Overview of Newton's Third Law of Motion

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  • 0:01 Pushing Back
  • 0:49 Newton's Third Law
  • 1:41 Examples of Push Back
  • 2:48 Understanding Normal Force
  • 3:29 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

Ever felt the recoil of a paintball gun? Or, hurt your hand after hitting a wall? Surely you've breathed, right? In this lesson, we explain Newton's third law of motion, which makes all of those possible.

Pushing Back

Have you ever played pool? Even if you can't cause a trick shot where balls seem to leap over others, pool tables are great places to see the laws of motion in practice. Unless your friend bumps the table, the balls all stay put until the slightest force acts upon them, just like Newton's first law of motion states. Meanwhile, hitting a ball harder causes it to move faster, proving Newton's second law.

But what about his third law? Simply put, Newton's third law of motion makes pool possible. It states that for every action there is an equal and opposite reaction. In other words, if not for Newton's third law, the balls simply would not move across the table, much less bank off the walls for a shot that impresses your buddies.

Newton's Third Law

Think about anything you do and there's a reaction to it. However, this isn't as simple as turning in your homework and getting an A. I'm talking complete science here. Let's say that your friend knocked in a great shot, effectively meaning that you lose the game. Out of sheer frustration, you punch the wall.

Sure, if you were really mad and really strong, you might very well punch a hole through the wall. However, chances are that your hand is not exactly pristine after the punch. The wall exerted a force equal and opposite to what you exerted on the wall. In fact, it came off at exactly the same angle and everything.

So, what would happen if the wall did not exert such a force on your hand? In that case, your hand would flow effortlessly through the wall and into the restaurant booth on the other side! However, if that were the case, you'd have much bigger problems. Even breathing would be impossible without Newton's third law.

Examples of Push Back

Take a deep breath. While it may feel like there's nothing in the air, there is still a definite mass to all the oxygen, nitrogen, and carbon dioxide molecules in the air we breathe. Now exhale. Your diaphragm pushes all the air out of your lungs; the diaphragm acts on the air, and the air acts on the diaphragm. However, the contracting space in your lungs means that all of that air is forced out. Had your throat not been open to allow you to exhale, the pressure in your lungs would build.

That's a pretty hard example to get, so let's try something with much more visible forces. Let's say that you were playing paintball, probably in an effort to get back at the friend who bested you in pool. You see him trying to sneak out from behind a barrel and you let loose a whole spray of paintballs. However, your paintball gun pushes back against your shoulder as you do. The paintball leaves the gun with a force, but the equal and opposite part of the force ends up pressing back against your shoulder.

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