essay meaning in physics

Introductory essay

Written by the educators who created The Edge of Knowledge, a brief look at the key facts, tough questions and big ideas in their field. Begin this TED Study with a fascinating read that gives context and clarity to the material.

Particle physicists are nothing if not ambitious. And the aim of particle physics is to understand what everything's made of, and how everything sticks together. And by everything I mean, of course, me and you, the Earth, the Sun, the 100 billion suns in our galaxy and the 100 billion galaxies in the observable universe. Absolutely everything. Brian Cox

To the outside observer, it may seem that physics is in some ways the opposite of art and that physicists must sacrifice their artistic intelligence to make way for cold rationality and logic. But nothing could be farther from the truth: Each step forward in our understanding of the universe could not have been conceived without an enormous dose of intuition and creativity.

Physicists are on a quest to figure out how nature works at the most fundamental level. This is a romantic story, penned in what may seem the least emotive of languages: mathematics. What's surprising is that the immeasurable beauty of the world is far from lost once its inner workings are expressed in this abstract language. Moreover, there remains something deeply intriguing about the fact that the universe is governed by the rules of mathematics in the first place. As we'll hear from Murray Gell-Mann in the first of the TEDTalks in The Edge of Knowledge, these beautiful mathematical laws are "not merely a conceit of the human mind" — instead, they're an intrinsic part of nature.

Many successful ideas in science can be described as beautiful and very often this is a reference to the simplicity and conciseness of nature's laws. Einstein's special and general theories of relativity, which describe how space, time and gravity behave, are based on only three brief postulates. The laws of electromagnetism, which govern every aspect of how we experience the worldthrough sight, sound, smell, taste or touch, are so concise thatthey can be written on the front of a T-shirt. The Standard Model of Particle Physics, which describes all of the known particles and three of the four forces that act between them, fits on the side of a coffee mug. As we will hear from Garrett Lisi, looking for beauty in the patterns that emerge in the laws of physics can tell us about how the universe works at the most fundamental level.

Science is a collaborative discipline and a global one too. It is the extent to which scientists cooperate that allows science to move at an incredible pace. The majority of the ideas presented in these TEDTalks have been around no longer than 50 years; some less than a decade. Since these speakers featured in The Edge of Knowledge delivered their TEDTalks, scientists working in global collaborations have developed and implemented several new experimental measurements. Most recently, the European Space Agency's Planck satellite has made precise measurements of the Cosmic Microwave Background (CMB): the results are in agreement with the predictions of the Standard Cosmological Model, which describes how the universe evolved from the Big Bang to what we see today. In Brian Cox's TEDTalk, we'll hear about the search for new elementary particles at CERN's Large Hadron Collider, encompassing the work of over 10,000 physicists from over 100 countries. This search is underway, and appears already to have yielded one of the most important scientific results of the 21st century: the discovery of the Higgs boson, the final ingredient predicted by the Standard Model of Particle Physics.

Notwithstanding the significance of these recent discoveries and their agreement with predictions, our picture of the fundamental structure of the universe is far from complete: a number of big mysteries remain in both particle physics and cosmology. As we'll hear from Patricia Burchat, many of these mysteries link together the physics of the smallest elementary particles and the largest distances of the cosmos. One of the most enduring mysteries is how to reconcile a complete theory of gravity with our understanding of the fundamental particles. From Brian Greene, we'll hear about the potential of string theory to solve this problem and the possible existence of tiny, curled up, extra spatial dimensions.

However, fundamental laws are not enough on their own. Aristotle said that "all human actions have one or more of these seven causes: chance, nature, compulsion, habit, reason, passion, and desire." It's the first of these — chance — that is not decided by the laws of physics; in fact, chance is not decided at all. The fundamental laws of physics cannot predict what will happen; they can only tell us what might happen. This uncertainty is built into the laws of quantum mechanics.

As we'll hear from Aaron O'Connell, the most striking feature of quantum mechanics is that it's weird. For example, we're challenged to contemplate the possibility that a thing can be in more than one place at the same time. It's quantum mechanics, more than any other idea in fundamental physics, which forces us to question our intuition about how everyday objects behave. For the microscopic constituents of the universe, our everyday observations simply do not hold. In spite of its counter-intuitiveness, quantum mechanics has come to define our modern world through the technologies that it underpins. From the tiny switches crammed by the billions onto microchips to medical scanners and laser therapies, all rely upon the weirdness of quantum mechanics.

Ultimately, science remains an empirical discipline. Thinking up beautiful theories is not enough on its own: every theory must stand up to the experimental observations of how nature actually works. If it doesn't, then the theory can't be correct and we must try again. If, on the other hand, our observations and predictions agree, then we're encouraged and — if the evidence is sufficient — we might even dare to claim some measure of understanding.

Through our human creativity, expressed in a process of trial and improvement, incremental advances in our understanding accumulate and scientific progress is made. As the chess grandmaster Gary Kasparov puts it, our success is "the ability to combine creativity and calculation...into a whole that is much greater than the sum of its parts." With each of these steps forward, science closes one more door and moves on to try one of the many, many doors that remain open.

This series of TEDTalks discusses some of the toughest questions and the most profound ideas in fundamental physics. The concepts not only challenge us to think objectively and rationally, but also require us to put aside many of our everyday preconceptions and intuitions about how nature works. Be prepared to re-watch the talks and re-read the supporting material; trying to get your head around 13.8 billion years of the universe's history isn't something you can do in an afternoon!

Let's begin with CalTech physicist Murray Gell-Mann for an introduction to the Standard Model of particle physics and the quest for a unified theory.

Murray Gell-Mann

Beauty, truth and ... physics, relevant talks.

Brian Greene

Making sense of string theory.

Garrett Lisi

An 8-dimensional model of the universe.

CERN's supercollider

Patricia Burchat

Shedding light on dark matter.

Aaron O'Connell

Making sense of a visible quantum object.

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Essays in Physics: Thirty-two thoughtful essays on topics in undergraduate-level physics

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“Essays in Physics” gives accounts of 32 chosen topics. The level is that of a 3–4-year university course in Physics. The topics discussed are diverse but “mainstream”. Each essay aims to say something fresh that complements what the reader will find elsewhere. Just what “fresh” means inevitably depends somewhat on the subject matter. Some chapters give a “different” slant on a familiar idea (e.g. electromagnetic energy, Lorentz transformation, photon emission). Some contain an analysis not available elsewhere (diffraction, feedback stability). Some correct material that is commonplace in many textbooks (much atomic physics). Some add insightful discussion to standard material (free energy, Brillouin zones). One in particular refines technique (perturbation theory). One brings order to confusion (- m d B ). The aim in all cases is to encourage a fuller, and correct, understanding, and an enhanced intellectual acuity (critical faculty). With a subject as mature as physics, it is bold to claim originality. However I will dare to make that claim, in particular for Chapters 10, 22 and 30, but also for parts of most other chapters.

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Why Study Physics?

The goal of physics is to understand how things work from first principles.  We offer physics courses that are matched to a range of goals that students may have in studying physics -- taking elective courses to broaden one's scientific literacy, satisfying requirements for a major in the sciences or engineering, or working towards a degree in physics or engineering physics. Courses in physics reveal the mathematical beauty of the universe at scales ranging from subatomic to cosmological. Studying physics strengthens quantitative reasoning and problem solving skills that are valuable in areas beyond physics.

Where do I start?

• Students who have never studied physics before and would like a broad introduction should consider one of the introductory seminar courses in Physics or Applied Physics. Those interested in astronomy and astrophysics might enjoy PHYSICS 15, 16 or 17, which is intended for nontechnical majors.
• Students considering a career in science or engineering should start with the PHYSICS 20 & 40 series or PHYSICS 61, 71, 81 .
• The PHYSICS 20 series assumes no background in calculus, and is intended primarily for those who are majoring in the biological sciences. However, such students who have AP credit in calculus or physics should consider taking the PHYSICS 40 series, which will provide a depth and emphasis on problem solving that is of significant value in biological research, which today involves considerable physics-based technology.
• For those intending to major in engineering or the physical sciences, or simply wishing a stronger background in physics, the department offers the PHYSICS 40 series and PHYSICS 61, 71, 81 . Either of these series will satisfy the entry-level physics requirements of any Stanford major.  However, students majoring in Physics or Engineering Physics are required to take PHYSICS 61, 71, 81 -- possibly after completing PHYSICS 41 and 43. 
• PHYSICS 61, 71, 81 courses are intended for those who have already taken a physics course at the level of PHYSICS 41 and 43, or at least have a strong background in mechanics, some background in electricity and magnetism, and a strong background in calculus. To determine whether you are prepared for PHYSICS 61, take the the Physics Placement Diagnostic .
• The PHYSICS 40 series begins with PHYSICS 41 (mechanics), which is offered as a 4-unit course in both Autumn and Winter quarters, and continues with PHYSICS 43 (electricity and magnetism) in both Winter and Spring quarters, and PHYSICS 45 (thermodynamics and optics) in Autumn quarter.
• Beginning in academic year 2023/2024, a five-unit version of PHYSICS 41 is offered in the Winter quarter: PHYSICS 41E (Extended). This course is designed to enable students who have had little or no high school physics background to succeed in physics. 
• The PHYSICS 61, 71, 81 series begins in the Autumn quarter (only) with special relativity and a deeper dive into mechanics.   
• While most students are recommended to begin with mechanics in the PHYSICS 40 series (PHYSICS 41 or 41E), those who have had strong physics preparation in high school (such as a score of at least 4 on the Physics Advanced Placement C exam) may be ready to start with PHYSICS 45 in Autumn quarter (and then take PHYSICS 43 in the Winter quarter), or to start with PHYSICS 61 in the Autumn. 
• Students are individually advised on the best entry point into either the PHYSICS 40 series or PHYSICS 61, 71, 81 on the basis of their score on the Physics Placement Diagnostic , which is available online.

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1: The Nature of Light

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In this chapter, we study the basic properties of light. In the next few chapters, we investigate the behavior of light when it interacts with optical devices such as mirrors, lenses, and apertures.

• 1.1: Prelude to The Nature of Light Maxwell’s equations predict the existence of electromagnetic waves and their behavior. Examples of light include radio and infrared waves, visible light, ultraviolet radiation, and X-rays. Interestingly, not all light phenomena can be explained by Maxwell’s theory. Experiments performed early in the twentieth century showed that light has corpuscular, or particle-like, properties.
• 1.2: The Propagation of Light The index of refraction of a material is \(n = \frac{c}{v}\), where v is the speed of light in a material and c is the speed of light in a vacuum. The ray model of light describes the path of light as straight lines. The part of optics dealing with the ray aspect of light is called geometric optics. Light can travel in three ways from a source to another location: (1) directly from the source through empty space; (2) through various media; and (3) after being reflected from a mirror.
• 1.3: The Law of Reflection By the end of this section, you will be able to: Explain the reflection of light from polished and rough surfaces. Describe the principle and applications of corner reflectors.
• 1.4: Refraction By the end of this section, you will be able to: Describe how rays change direction upon entering a medium. Apply the law of refraction in problem solving
• 1.5: Total Internal Reflection By the end of this section, you will be able to: Explain the phenomenon of total internal reflection. Describe the workings and uses of optical fibers. Analyze the reason for the sparkle of diamonds
• 1.6: Dispersion By the end of this section, you will be able to: Explain the cause of dispersion in a prism. Describe the effects of dispersion in producing rainbows. Summarize the advantages and disadvantages of dispersion
• 1.7: Huygens’s Principle some phenomena require analysis and explanations based on the wave characteristics of light. This is particularly true when the wavelength is not negligible compared to the dimensions of an optical device, such as a slit in the case of diffraction. Huygens’s principle is an indispensable tool for this analysis. For example, according to Huygens’s principle, every point on a wave front is a source of wavelets that spread out in the forward direction at the same speed as the wave itself.
• 1.8: Polarization Polarization is the attribute that wave oscillations have a definite direction relative to the direction of propagation of the wave. The direction of polarization is defined to be the direction parallel to the electric field of the EM wave. Unpolarized light is composed of many rays having random polarization directions. Unpolarized light can be polarized by passing it through a polarizing filter or other polarizing material. The process of polarizing light decreases its intensity by a factor of
• 1.A: The Nature of Light (Answers)
• 1.E: The Nature of Light (Exercises)
• 1.S: The Nature of Light (Summary)

Thumbnail: An EM wave, such as light, is a transverse wave. The electric \(\overrightarrow{E}\) and magnetic \(\overrightarrow{B}\) fields are perpendicular to the direction of propagation. The direction of polarization of the wave is the direction of the electric field.

What Is Quantum Physics?

This article was reviewed by a member of Caltech's Faculty .

Quantum physics is the study of matter and energy at the most fundamental level. It aims to uncover the properties and behaviors of the very building blocks of nature.

While many quantum experiments examine very small objects, such as electrons and photons, quantum phenomena are all around us, acting on every scale. However, we may not be able to detect them easily in larger objects. This may give the wrong impression that quantum phenomena are bizarre or otherworldly. In fact, quantum science closes gaps in our knowledge of physics to give us a more complete picture of our everyday lives.

Quantum discoveries have been incorporated into our foundational understanding of materials, chemistry, biology, and astronomy. These discoveries are a valuable resource for innovation, giving rise to devices such as lasers and transistors, and enabling real progress on technologies once considered purely speculative, such as quantum computers . Physicists are exploring the potential of quantum science to transform our view of gravity and its connection to space and time. Quantum science may even reveal how everything in the universe (or in multiple universes) is connected to everything else through higher dimensions that our senses cannot comprehend.

The Origins of Quantum Physics

The field of quantum physics arose in the late 1800s and early 1900s from a series of experimental observations of atoms that didn't make intuitive sense in the context of classical physics. Among the basic discoveries was the realization that matter and energy can be thought of as discrete packets, or quanta, that have a minimum value associated with them. For example, light of a fixed frequency will deliver energy in quanta called "photons." Each photon at this frequency will have the same amount of energy, and this energy can't be broken down into smaller units. In fact, the word "quantum" has Latin roots and means "how much."

Knowledge of quantum principles transformed our conceptualization of the atom, which consists of a nucleus surrounded by electrons. Early models depicted electrons as particles that orbited the nucleus, much like the way satellites orbit Earth. Modern quantum physics instead understands electrons as being distributed within orbitals, mathematical descriptions that represent the probability of the electrons' existence in more than one location within a given range at any given time. Electrons can jump from one orbital to another as they gain or lose energy, but they cannot be found between orbitals.

Other central concepts helped to establish the foundations of quantum physics:

• Wave-particle duality: This principle dates back to the earliest days of quantum science. It describes the outcomes of experiments that showed that light and matter had the properties of particles or waves, depending on how they were measured. Today, we understand that these different forms of energy are actually neither particle nor wave. They are distinct quantum objects that we cannot easily conceptualize.
• Superposition : This is a term used to describe an object as a combination of multiple possible states at the same time. A superposed object is analogous to a ripple on the surface of a pond that is a combination of two waves overlapping. In a mathematical sense, an object in superposition can be represented by an equation that has more than one solution or outcome.
• Uncertainty principle : This is a mathematical concept that represents a trade-off between complementary points of view. In physics, this means that two properties of an object, such as its position and velocity, cannot both be precisely known at the same time. If we precisely measure the position of an electron, for example, we will be limited in how precisely we can know its speed.
• Entanglement : This is a phenomenon that occurs when two or more objects are connected in such a way that they can be thought of as a single system, even if they are very far apart. The state of one object in that system can't be fully described without information on the state of the other object. Likewise, learning information about one object automatically tells you something about the other and vice versa.

Mathematics and the Probabilistic Nature of Quantum Objects

Because many of the concepts of quantum physics are difficult if not impossible for us to visualize, mathematics is essential to the field. Equations are used to describe or help predict quantum objects and phenomena in ways that are more exact than what our imaginations can conjure.

Mathematics is also necessary to represent the probabilistic nature of quantum phenomena. For example, the position of an electron may not be known exactly. Instead, it may be described as being in a range of possible locations (such as within an orbital), with each location associated with a probability of finding the electron there.

Given their probabilistic nature, quantum objects are often described using mathematical "wave functions," which are solutions to what is known as the Schrödinger equation . Waves in water can be characterized by the changing height of the water as the wave moves past a set point. Similarly, sound waves can be characterized by the changing compression or expansion of air molecules as they move past a point. Wave functions don't track with a physical property in this way. The solutions to the wave functions provide the likelihoods of where an observer might find a particular object over a range of potential options. However, just as a ripple in a pond or a note played on a trumpet are spread out and not confined to one location, quantum objects can also be in multiple places—and take on different states, as in the case of superposition—at once.

Observation of Quantum Objects

The act of observation is a topic of considerable discussion in quantum physics. Early in the field, scientists were baffled to find that simply observing an experiment influenced the outcome. For example, an electron acted like a wave when not observed, but the act of observing it caused the wave to collapse (or, more accurately, "decohere") and the electron to behave instead like a particle. Scientists now appreciate that the term "observation" is misleading in this context, suggesting that consciousness is involved. Instead, "measurement" better describes the effect, in which a change in outcome may be caused by the interaction between the quantum phenomenon and the external environment, including the device used to measure the phenomenon. Even this connection has caveats, though, and a full understanding of the relationship between measurement and outcome is still needed.

The Double-Slit Experiment

Perhaps the most definitive experiment in the field of quantum physics is the double-slit experiment . This experiment, which involves shooting particles such as photons or electrons through a barrier with two slits, was originally used in 1801 to show that light is made up of waves. Since then, numerous incarnations of the experiment have been used to demonstrate that matter can also behave like a wave and to demonstrate the principles of superposition, entanglement, and the observer effect.

The field of quantum science may seem mysterious or illogical, but it describes everything around us, whether we realize it or not. Harnessing the power of quantum physics gives rise to new technologies, both for applications we use today and for those that may be available in the future .

Dive Deeper

Why Are Soap Bubbles So Colorful? Iridescence explained by wave theory and quantum electrodynamics

Charlie Marcus Knows That Quantum Facts Aren't Complicated

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What is physics essay sample, example.

Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the respective positions of the things which compose it… nothing would be uncertain, and the future as the past would be laid out before its eyes.

– Pierre Simon de Laplace

Physics is the use of the scientific method to find out the basic principles governing light and matter, and to discover the implications of those laws. Part of what distinguishes the modern outlook from the ancient mindset is the assumption that there are rules by which the universe functions, and that those laws can be at least partially understood by humans. From the Age of Reason through the nineteenth century, many scientists began to be convinced that the laws of nature not only could be known but, as claimed by Laplace, those laws could in principle be used to predict everything about the universe’s future if complete information was available about the present state of all light and matter.

Matter can be defined as anything affected by gravity, i.e., that has weight or would have weight if it was near Earth or another star or planet massive enough to produce measurable gravity. Light can be defined as anything that can travel from one place to another through empty space and can influence matter, but has no weight. For example, sunlight can influence your body by heating it or by damaging your DNA and giving you skin cancer. The physicist’s definition of light includes a variety of phenomena that are not visible to the eye, including radio waves, microwaves, x-rays, and gamma rays. These are the “colors” of light that do not happen to fall within the narrow violet-to-red range of the rainbow that we can see.

Many physical phenomena are not themselves light or matter, but are properties of light or matter or interactions between light and matter. For instance, motion is a property of all light and some matter, but it is not itself light or matter. The pressure that keeps a bicycle tire blown up is an interaction between the air and the tire. Pressure is not a form of matter in and of itself. It is as much a property of the tire as of the air. Analogously, sisterhood and employment are relationships among people but are not people themselves.

Some things that appear weightless actually do have weight, and so qualify as matter. Air has weight, and is thus a form of matter even though a cubic inch of air weighs less than a grain of sand. A helium balloon has weight, but is kept from falling by the force of the surrounding more dense air, which pushes up on it. Astronauts in orbit around Earth have weight, and are falling along a curved arc, but they are moving so fast that the curved arc of their fall is broad enough to carry them all the way around Earth in a circle. They perceive themselves as being weightless because their space capsule is falling along with them, and the floor therefore does not push up on their feet.

Einstein predicted as a consequence of his theory of relativity that light would after all be affected by gravity, although the effect would be extremely weak under normal conditions. His prediction was borne out by observations of the bending of light rays from stars as they passed close to the sun on their way to Earth. Einstein’s theory also implied the existence of black holes, stars so massive and compact that their intense gravity would not even allow light to escape (these days, there is strong evidence that black holes exist).

Einstein’s interpretation was that light does not really have mass, but that energy is affected by gravity just like mass is. The energy in a light beam is equivalent to a certain amount of mass, given by the famous equation E=mc2, where c is the speed of light. Because the speed of light is such a big number, a large amount of energy is equivalent to only a very small amount of mass, so the gravitational force on a light ray can be ignored for most practical purposes.

There is however a more satisfactory and fundamental distinction between light and matter, which should be understandable to you if you have had a chemistry course. In chemistry, one learns that electrons obey the Pauli exclusion principle, which forbids more than one electron from occupying the same orbital if they have the same spin. The Pauli exclusion principle is obeyed by the subatomic particles of which matter is composed, but disobeyed by the particles, called photons, of which a beam of light is made.

The boundary between physics and the other sciences is not always clear. For instance, chemists study atoms and molecules, which are what matter is built from, and there are some scientists who would be equally willing to call themselves physical chemists or chemical physicists. It might seem that the distinction between physics and biology would be clearer, since physics seems to deal with inanimate objects. In fact, almost all physicists would agree that the basic laws of physics that apply to molecules in a test tube work equally well for the combination of molecules that constitutes a bacterium (some might believe that something more happens in the minds of humans, or even those of cats and dogs). What differentiates physics from biology is that many of the scientific theories that describe living things, while ultimately resulting from the fundamental laws of physics, cannot be rigorously derived from physical principles.

Isolated Systems and Reductionism

To avoid having to study everything at once, scientists isolate the things they are trying to study. For instance, a physicist who wants to study the motion of a rotating gyroscope would probably prefer that it be isolated from vibrations and air currents. Even in biology, where field work is indispensable for understanding how living things relate to their entire environment, it is interesting to note the vital historical role played by Darwin’s study of the Galápagos Islands, which were conveniently isolated from the rest of the world. Any part of the universe that is considered apart from the rest can be called a “system.”

Physics has had some of its greatest successes by carrying this process of isolation to extremes, subdividing the universe into smaller and smaller parts. Matter can be divided into atoms, and the behavior of individual atoms can be studied. Atoms can be split apart into their constituent neutrons, protons, and electrons. Protons and neutrons appear to be made out of even smaller particles called quarks, and there have even been some claims of experimental evidence that quarks have smaller parts inside them. This method of splitting things into smaller and smaller parts and studying how those parts influence each other is called reductionism. The hope is that the seemingly complex rules governing the larger units can be better understood in terms of simpler rules governing the smaller units. To appreciate what reductionism has done for science, it is only necessary to examine a 19th-century chemistry textbook. At that time, the existence of atoms was still doubted by some, electrons were not even suspected to exist, and almost nothing was understood of what basic rules governed the way atoms interacted with each other in chemical reactions. Students had to memorize long lists of chemicals and their reactions, and there was no way to understand any of it systematically. Today, the student only needs to remember a small set of rules about how atoms interact, for instance that atoms of one element cannot be converted into another via chemical reactions, or that atoms from the right side of the periodic table tend to form strong bonds with atoms from the left side.

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• Introduction To Motion
• Motion In Physics

Motion in Physics

The concept of motion in physics is one of the important topics in Classical Mechanics. Did you know that everything in the universe is always moving? Even if you are completely still, you still belong to the earth which is continuously moving about its axis and around the sun. Motion means a change in the position of an object with reference to time.

What Is Motion in Physics?

In physics, motion is the change in position of an object with respect to its surroundings in a given interval of time. The motion of an object with some mass can be described in terms of the following:

• Displacement
• Acceleration

Watch the video and learn more about laws of motion

Types of Motion in Physics

The motion of an object depends on the type of force acting on the body. Examples of different kinds of motion are given below.

• Translational  – It is the type, where an object moves along a path in any of the three dimensions.
• Rotational  – It is the type, where an object moves along a circular path about a fixed axis.
• Linear  – It is a type of translational motion where the body moves in a single direction along a single dimension.
• Periodic  – It is the type of motion that repeats itself after certain intervals of time
• Simple Harmonic  – It is the type of motion like that of a simple pendulum where a restoring force acts in the direction opposite to the direction of motion of the object. This restoring force is proportional to the displacement of the object from the mean position.
• Projectile – It is the type of motion which has a horizontal displacement as well as vertical displacement.
• Oscillatory  – It is the type of motion which is repetitive in nature within a time frame. If it is mechanical it is called vibration.

Read More: Laws of Motion

Laws of Motion

Newton’s Laws of Motion laid the foundation for classical mechanics today. Although subject to minor limitations, these laws of motion are valid everywhere and are therefore used. The laws are given as stated below in a brief description

• First Law : Any object will remain in its existing state of motion or rest unless a net external force acts on it.
• Second Law : If an object has a certain mass, the greater the mass of this object, the greater will the force required to be to accelerate the object. It is represented by the equation F = ma, where ‘F’ is the force on the object, ‘m’ is the mass of the object and ‘a’ is the acceleration of the object.
• Third Law : For every action, there is an equal and opposite reaction.

The below video provides the Top 10 NTSE Important Questions on Motion Class 9

Frequently Asked Questions – FAQs

What is periodic motion, what is rotational motion, what is newton’s first law of motion, state newton’s third law of motion., what is oscillatory motion, the below video provides the complete quiz of the chapter motion class 9.

Motion is an important concept in Physics which can be better understood by applying conceptual knowledge to solve problems. Stay tuned with BYJU’S and learn various interesting physics topics with the help of engaging video lessons.

Put your understanding of this concept to test by answering a few MCQs. Click ‘Start Quiz’ to begin!

Select the correct answer and click on the “Finish” button Check your score and answers at the end of the quiz

Visit BYJU’S for all Physics related queries and study materials

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Physics library

Course: physics library   >   unit 1.

• Acceleration

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5.3 Projectile Motion

Section learning objectives.

By the end of this section, you will be able to do the following:

• Describe the properties of projectile motion
• Apply kinematic equations and vectors to solve problems involving projectile motion

Teacher Support

The learning objectives in this section will help your students master the following standards:

• (C) analyze and describe accelerated motion in two dimensions using equations.

In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled: Motion in Two Dimensions, as well as the following standards:

• (C) analyze and describe accelerated motion in two dimensions using equations, including projectile and circular examples.

Section Key Terms

Properties of projectile motion.

Projectile motion is the motion of an object thrown (projected) into the air when, after the initial force that launches the object, air resistance is negligible and the only other force that object experiences is the force of gravity. The object is called a projectile , and its path is called its trajectory . Air resistance is a frictional force that slows its motion and can significantly alter the trajectory of the motion. Due to the difficulty in calculation, only situations in which the deviation from projectile motion is negligible and air resistance can be ignored are considered in introductory physics. That approximation is often quite accurate.

[BL] [OL] Review addition of vectors graphically and analytically.

[BL] [OL] [AL] Explain the term projectile motion. Ask students to guess what the motion of a projectile might depend on? Is the initial velocity important? Is the angle important? How will these things affect its height and the distance it covers? Introduce the concept of air resistance. Review kinematic equations.

The most important concept in projectile motion is that when air resistance is ignored, horizontal and vertical motions are independent , meaning that they don’t influence one another. Figure 5.27 compares a cannonball in free fall (in blue) to a cannonball launched horizontally in projectile motion (in red). You can see that the cannonball in free fall falls at the same rate as the cannonball in projectile motion. Keep in mind that if the cannon launched the ball with any vertical component to the velocity, the vertical displacements would not line up perfectly.

Since vertical and horizontal motions are independent, we can analyze them separately, along perpendicular axes. To do this, we separate projectile motion into the two components of its motion, one along the horizontal axis and the other along the vertical.

We’ll call the horizontal axis the x -axis and the vertical axis the y -axis. For notation, d is the total displacement, and x and y are its components along the horizontal and vertical axes. The magnitudes of these vectors are x and y , as illustrated in Figure 5.28 .

As usual, we use velocity, acceleration, and displacement to describe motion. We must also find the components of these variables along the x - and y -axes. The components of acceleration are then very simple a y = – g = –9.80 m/s 2 . Note that this definition defines the upwards direction as positive. Because gravity is vertical, a x = 0. Both accelerations are constant, so we can use the kinematic equations. For review, the kinematic equations from a previous chapter are summarized in Table 5.1 .

Where x is position, x 0 is initial position, v is velocity, v avg is average velocity, t is time and a is acceleration.

Solve Problems Involving Projectile Motion

The following steps are used to analyze projectile motion:

• Separate the motion into horizontal and vertical components along the x- and y-axes. These axes are perpendicular, so A x = A cos θ A x = A cos θ and A y = A sin θ A y = A sin θ are used. The magnitudes of the displacement s s along x- and y-axes are called x x and y . y . The magnitudes of the components of the velocity v v are v x = v ​ ​ ​ cos θ v x = v ​ ​ ​ cos θ and v y = v ​ ​ ​ sin θ v y = v ​ ​ ​ sin θ , where v v is the magnitude of the velocity and θ θ is its direction. Initial values are denoted with a subscript 0.
• Treat the motion as two independent one-dimensional motions, one horizontal and the other vertical. The kinematic equations for horizontal and vertical motion take the following forms Horizontal Motion ( a x = 0 ) x = x 0 + v x t v x = v 0 x = v x = velocity  is a constant. Horizontal Motion ( a x = 0 ) x = x 0 + v x t v x = v 0 x = v x = velocity  is a constant. Vertical motion (assuming positive is up a y = − g = − 9.80  m/s 2 a y = − g = − 9.80  m/s 2 ) y = y 0 + 1 2 ( v 0 y + v y ) t v y = v 0 y − g t y = y 0 + v 0 y t − 1 2 g t 2 v y 2 = v 0 y 2 − 2 g ( y − y 0 ) y = y 0 + 1 2 ( v 0 y + v y ) t v y = v 0 y − g t y = y 0 + v 0 y t − 1 2 g t 2 v y 2 = v 0 y 2 − 2 g ( y − y 0 )
• Solve for the unknowns in the two separate motions (one horizontal and one vertical). Note that the only common variable between the motions is time t t . The problem solving procedures here are the same as for one-dimensional kinematics.

Teacher Demonstration

Demonstrate the path of a projectile by doing a simple demonstration. Toss a dark beanbag in front of a white board so that students can get a good look at the projectile path. Vary the toss angles, so different paths can be displayed. This demonstration could be extended by using digital photography. Draw a reference grid on the whiteboard, then toss the bag at different angles while taking a video. Replay this in slow motion to observe and compare the altitudes and trajectories.

Tips For Success

For problems of projectile motion, it is important to set up a coordinate system. The first step is to choose an initial position for x x and y y . Usually, it is simplest to set the initial position of the object so that x 0 = 0 x 0 = 0 and y 0 = 0 y 0 = 0 .

Watch Physics

Projectile at an angle.

This video presents an example of finding the displacement (or range) of a projectile launched at an angle. It also reviews basic trigonometry for finding the sine, cosine and tangent of an angle.

• The time to reach the ground would remain the same since the vertical component is unchanged.
• The time to reach the ground would remain the same since the vertical component of the velocity also gets doubled.
• The time to reach the ground would be halved since the horizontal component of the velocity is doubled.
• The time to reach the ground would be doubled since the horizontal component of the velocity is doubled.

Worked Example

A fireworks projectile explodes high and away.

During a fireworks display like the one illustrated in Figure 5.30 , a shell is shot into the air with an initial speed of 70.0 m/s at an angle of 75° above the horizontal. The fuse is timed to ignite the shell just as it reaches its highest point above the ground. (a) Calculate the height at which the shell explodes. (b) How much time passed between the launch of the shell and the explosion? (c) What is the horizontal displacement of the shell when it explodes?

The motion can be broken into horizontal and vertical motions in which a x = 0 a x = 0 and   a y = g   a y = g . We can then define x 0 x 0 and y 0 y 0 to be zero and solve for the maximum height .

By height we mean the altitude or vertical position y y above the starting point. The highest point in any trajectory, the maximum height, is reached when   v y = 0   v y = 0 ; this is the moment when the vertical velocity switches from positive (upwards) to negative (downwards). Since we know the initial velocity, initial position, and the value of v y when the firework reaches its maximum height, we use the following equation to find y y

Because y 0 y 0 and v y v y are both zero, the equation simplifies to

Solving for y y gives

Now we must find v 0 y v 0 y , the component of the initial velocity in the y -direction. It is given by v 0 y = v 0 sin θ v 0 y = v 0 sin θ , where v 0 y v 0 y is the initial velocity of 70.0 m/s, and θ = 75 ∘ θ = 75 ∘ is the initial angle. Thus,

Since up is positive, the initial velocity and maximum height are positive, but the acceleration due to gravity is negative. The maximum height depends only on the vertical component of the initial velocity. The numbers in this example are reasonable for large fireworks displays, the shells of which do reach such heights before exploding.

There is more than one way to solve for the time to the highest point. In this case, the easiest method is to use y = y 0 + 1 2 ( v 0 y + v y ) t y = y 0 + 1 2 ( v 0 y + v y ) t . Because y 0 y 0 is zero, this equation reduces to

Note that the final vertical velocity, v y v y , at the highest point is zero. Therefore,

This time is also reasonable for large fireworks. When you are able to see the launch of fireworks, you will notice several seconds pass before the shell explodes. Another way of finding the time is by using y = y 0 + v 0 y t − 1 2 g t 2 y = y 0 + v 0 y t − 1 2 g t 2 , and solving the quadratic equation for t t .

Because air resistance is negligible, a x = 0 a x = 0 and the horizontal velocity is constant. The horizontal displacement is horizontal velocity multiplied by time as given by x = x 0 + v x t x = x 0 + v x t , where x 0 x 0 is equal to zero

where v x v x is the x -component of the velocity, which is given by v x = v 0 cos θ 0 . v x = v 0 cos θ 0 . Now,

The time t t for both motions is the same, and so x x is

The horizontal motion is a constant velocity in the absence of air resistance. The horizontal displacement found here could be useful in keeping the fireworks fragments from falling on spectators. Once the shell explodes, air resistance has a major effect, and many fragments will land directly below, while some of the fragments may now have a velocity in the –x direction due to the forces of the explosion.

[BL] [OL] [AL] Talk about the sample problem. Discuss the variables or unknowns in each part of the problem Ask students which kinematic equations may be best suited to solve the different parts of the problem.

The expression we found for y y while solving part (a) of the previous problem works for any projectile motion problem where air resistance is negligible. Call the maximum height y = h y = h ; then,

This equation defines the maximum height of a projectile . The maximum height depends only on the vertical component of the initial velocity.

Calculating Projectile Motion: Hot Rock Projectile

Suppose a large rock is ejected from a volcano, as illustrated in Figure 5.31 , with a speed of 25.0   m / s 25.0   m / s and at an angle 3 5 ° 3 5 ° above the horizontal. The rock strikes the side of the volcano at an altitude 20.0 m lower than its starting point. (a) Calculate the time it takes the rock to follow this path.

Breaking this two-dimensional motion into two independent one-dimensional motions will allow us to solve for the time. The time a projectile is in the air depends only on its vertical motion.

While the rock is in the air, it rises and then falls to a final position 20.0 m lower than its starting altitude. We can find the time for this by using

If we take the initial position y 0 y 0 to be zero, then the final position is y = − 20.0  m . y = − 20.0  m . Now the initial vertical velocity is the vertical component of the initial velocity, found from

Substituting known values yields

Rearranging terms gives a quadratic equation in t t

This expression is a quadratic equation of the form a t 2 + b t + c = 0 a t 2 + b t + c = 0 , where the constants are a = 4.90, b = –14.3, and c = –20.0. Its solutions are given by the quadratic formula

This equation yields two solutions t = 3.96 and t = –1.03. You may verify these solutions as an exercise. The time is t = 3.96 s or –1.03 s. The negative value of time implies an event before the start of motion, so we discard it. Therefore,

The time for projectile motion is completely determined by the vertical motion. So any projectile that has an initial vertical velocity of 14.3 m / s 14.3 m / s and lands 20.0 m below its starting altitude will spend 3.96 s in the air.

Practice Problems

The fact that vertical and horizontal motions are independent of each other lets us predict the range of a projectile. The range is the horizontal distance R traveled by a projectile on level ground, as illustrated in Figure 5.32 . Throughout history, people have been interested in finding the range of projectiles for practical purposes, such as aiming cannons.

How does the initial velocity of a projectile affect its range? Obviously, the greater the initial speed v 0 v 0 , the greater the range, as shown in the figure above. The initial angle θ 0 θ 0 also has a dramatic effect on the range. When air resistance is negligible, the range R R of a projectile on level ground is

where v 0 v 0 is the initial speed and θ 0 θ 0 is the initial angle relative to the horizontal. It is important to note that the range doesn’t apply to problems where the initial and final y position are different, or to cases where the object is launched perfectly horizontally.

Virtual Physics

Projectile motion.

In this simulation you will learn about projectile motion by blasting objects out of a cannon. You can choose between objects such as a tank shell, a golf ball or even a Buick. Experiment with changing the angle, initial speed, and mass, and adding in air resistance. Make a game out of this simulation by trying to hit the target.

Check Your Understanding

• Projectile motion is the motion of an object projected into the air and moving under the influence of gravity.
• Projectile motion is the motion of an object projected into the air and moving independently of gravity.
• Projectile motion is the motion of an object projected vertically upward into the air and moving under the influence of gravity.
• Projectile motion is the motion of an object projected horizontally into the air and moving independently of gravity.

What is the force experienced by a projectile after the initial force that launched it into the air in the absence of air resistance?

• The nuclear force
• The gravitational force
• The electromagnetic force
• The contact force

Use the Check Your Understanding questions to assess whether students achieve the learning objectives for this section. If students are struggling with a specific objective, the Check Your Understanding will help identify which objective is causing the problem and direct students to the relevant content.

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• Authors: Paul Peter Urone, Roger Hinrichs
• Publisher/website: OpenStax
• Book title: Physics
• Publication date: Mar 26, 2020
• Location: Houston, Texas
• Book URL: https://openstax.org/books/physics/pages/1-introduction
• Section URL: https://openstax.org/books/physics/pages/5-3-projectile-motion

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