Learn about the basic principles that govern the physical world around us. Solid understanding of algebra and a basic understanding of trigonometry necessary.
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One-dimensional motion

In this tutorial we begin to explore ideas of velocity and acceleration. We do exciting things like throw things off cliffs (far safer on paper than in real life) and see how high a ball will fly in the air.

Two-dimensional motion

You understand velocity and acceleration well in one-dimension. Now we can explore scenarios that are even more fun. With a little bit of trigonometry (you might want to review your basic trig, especially what sin and cos are), we can think about whether a baseball can clear the "green monster" at Fenway Park.

Forces and Newton's laws of motion

This is the meat of much of classical physics. We think about what a force is and how Newton changed the world's (and possibly your) view of how reality works.

Work and energy

"Energy" is a word that's used a lot. Here, you'll learn about how it's one of the most useful concepts in physics. Along the way, we'll talk about work, kinetic energy, potential energy, conservation of energy, and mechanical advantage.

Impacts and linear momentum

Momentum ties velocity and mass into one quantity. It might not be obvious why this is useful, but momentum has this cool property where the total amount of it never changes. This is called the conservation of momentum, and we can use it to analyze collisions and other interactions. Bam!

Moments, torque, and angular momentum

Everything you've learned about motion, forces, energy, and momentum can be reused to analyze rotating objects. There are some differences, though. Here, you'll learn about rotational motion, moments, torque, and angular momentum.


Gravity is the force of attraction between masses. It's the thing that pulls you down to earth. Here, you'll learn precise meanings of the words mass and weight, and you'll also learn how gravity affects falling near earth and orbits in space.

Oscillatory motion

Pendulums. Slinkies. You when you have to use the bathroom but it is occupied. These all go back and forth over and over and over again. This tutorial explores this type of motion.


Atmospheric pressure is like an invisible friend who is always squeezing you with a big hug. Learn more about pressure, buoyant force, and flowing fluid so you can appreciate the sometimes invisible, but crucial, effect they have on us and the world around us.


Heat can be useful, but it can also be annoying. Understanding heat and the flow of heat allows us to build heat sinks that prevent our computers from overheating, build better engines, and prevent freeway overpasses from cracking.

Electric charge, electric force, and voltage

Electric forces hold together the atoms and molecules in your eyes which allow you to read this sentence. Take a moment and learn about the force that holds our bodies together.


Circuits make computers, digital cameras, and video games possible. Circuits are driving an unprecedented rate of change in how we live. In this topic you'll learn about the physics behind the electronic devices we use.

Magnetic forces and magnetic fields

The magnetic field of the Earth shields us from harmful radiation from the Sun. Magnetic fields also allow us to diagnose medical problems using an MRI. In this topic you'll learn about the force and field that makes this possible.

Magnetic flux and Faraday's law

Faraday's law is how we get electrical power from most power plants and hydroelectric dams. Learn how magnetic flux allows us to turn the mechanical energy of falling water through a dam into electrical energy.

Mechanical waves and sound

Waves are responsible for basically every form of communication we use. Whether you're talking out loud, texting on your phone, or waving to someone in a crowd there's going to be a wave transmitting information. Learn about the basics of waves in this topic, then learn more about light waves in the topics below.

Light waves

Light can seem mysterious. What is light made out of? What causes color? How do 3D movies work? Learn about some of the mysterious properties of light in these tutorials.

Geometric optics

Light waves can be bent and reflected to form new and sometimes altered images. Understanding how light rays can be manipulated allows us to create better contact lenses, fiber optic cables, and high powered telescopes.

Community explanations

Calling great explainers of the world! Help us answer frequently asked physics questions by writing clear, deep, engaging explanations. Your explanation may even be selected for an article on Khan Academy (with full credit to you)!

Ask a Physicist! (archived)

Hi everyone, I'm David, and I love physics. Have a physics question/comment? Ask me anything. I'll be checking the comments section every couple hours until Sunday August 2nd at 12:00 PM PST and replying as often as possible. Also, please feel free to answer other people's questions if you know the answer.

Forces and Newton's laws of motion

This is the meat of much of classical physics. We think about what a force is and how Newton changed the world's (and possibly your) view of how reality works.
Community Questions
All content in “Forces and Newton's laws of motion”

Normal force and contact force

A dog is balancing on one arm on my head. Is my head applying a force to the dog's hand? If it weren't, wouldn't there be nothing to offset the pull of gravity causing the acrobatic dog to fall? What would we call this force? Can we have a general term from the component of a contact force that acts perpendicular to the plane of contact? These are absolutely normal questions to ask.

Balanced and unbalanced forces

You will often hear physics professors be careful to say "net force" or "unbalanced force" rather than just "force". Why? This tutorial explains why and might give you more intuition about Newton's laws in the process.

Slow sock on Lubricon VI

This short tutorial will have you dealing with orbiting frozen socks in order to understand whether you understand Newton's Laws. We also quiz you a bit during the videos just to make sure that you aren't daydreaming about what you would do with a frozen sock.

Inclined planes and friction

We've all slid down slides/snow-or-mud-covered-hills/railings at some point in our life (if not, you haven't really lived) and noticed that the smoother the surface the more we would accelerate (try to slide down a non-snow-or-mud-covered hill). This tutorial looks into this in some depth. We'll look at masses on inclined planes and think about static and kinetic friction.


Bad commute? Baby crying? Bills to pay? Looking to take a bath with some Calgon (do a search on YouTube for context) to ease your tension? This tutorial has nothing (actually little, not nothing) to do with that. So far, most of the forces we've been dealing with are forces of "pushing"--contact forces at the macro level because of atoms not wanting to get to close at the micro level. Now we'll deal with "pulling" force or tension (at a micro level this is the force of attraction between bonded atoms).

Treating systems

When two or more objects must move with the same magnitude of acceleration (like masses on strings, or boxes pushed into each other), we can treat the entire system as a single object when finding the acceleration.