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Identifying Action and Reaction Force Pairs

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  • 0:03 Forces Come in Pairs
  • 1:27 Identifying Action & Reaction
  • 2:21 Different Objects Only
  • 3:21 The Effect of the Forces
  • 5:31 Action Equals Reaction
  • 6:32 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Good things often come in pairs, and forces are no exception. In this lesson you'll explore Newton's second and third laws of motion to understand how action and reaction pairs affect objects interacting with each other.

Forces Come in Pairs

Peanut butter and jelly. Thunder and lightning. Shoes and socks. Movies and popcorn. What do all these things have in common? They're undeniably awesome pairs. It's hard to imagine one without the other because they just go together so well. The same goes for forces - they come in pairs too! When one force acts there's an equal and opposite force acting against it because forces are the result of an interaction between objects.

Some of these interactions involve actual physical contact, like a baseball bat hitting a baseball and sending it flying. Other interactions are non-contact, like gravity pulling you down toward the earth as you jump from a diving board.

Regardless, the forces involved in an interaction follow Newton's third law of motion, which states that for every action force there is an opposed and equal reaction force. What this means is that when one object exerts a force on another object, that second object exerts an equal magnitude and opposite-direction force back on the first.

It doesn't really matter which force you call the action or reaction. What's most important to understand is that they go together, like Bert and Ernie, yin and yang, and rock and roll. And, these two forces are always equal in strength and opposite in direction.

Identifying Action and Reaction

It's fairly simple to identify the action and reaction forces between objects. First, it helps to identify the interaction itself. Let's say you are doing a push-up the floor. Or, maybe a tennis racket is hitting a tennis ball. How about a rocket launching off the ground?

All of these examples are interactions that result in forces being exerted on the objects involved in the interaction. When you push against the floor, you exert an action force onto the ground, while the ground exerts a reaction force right back on you.

In a tennis match, the racket exerts the action force on the ball and, as the ball hits it, it exerts an equal and opposite reaction force on the racket.

The rocket launches because it pushes on the gas coming out the back end for the action force, while the gas pushes the rocket upward with a reaction force.

Different Objects Only

In each of these cases, the action is the force exerted by the first object on the second, and the reaction force is the force exerted on the first object by the second. The important thing to notice is that action and reaction forces act on different objects in the interaction. Two equal and opposite forces acting on the same object do not make an action-reaction pair!

This is why the two opposing forces don't cancel out. If a football is kicked from both sides at the same time, it doesn't go anywhere, because the two feet kicking it exert equal and opposite forces on the same ball and cancel each other out.

However, when you kick a football with just one foot, that foot exerts an action force on the ball and, at the same time, the ball exerts a reaction force on your foot. In this case, the forces don't cancel out, so the ball travels through the air.

The Effect of the Forces

You may be wondering why the ball takes off but you stay standing on the ground. This has to do with the mass of the objects in the interaction. You are far more massive than the ball, so the ball accelerates. Both objects exert the same amount of force, but what's different is the effect of the forces on the different objects.

Let's take a closer look at this. Think of a cannon firing a cannonball. There is an interaction between the two objects, so there are action and reaction forces. However, the cannon recoils only slightly as it shoots out the cannonball, but the cannonball goes flying through the air.

We can look at Newton's second law of motion to see why this happens. This law states that acceleration is proportional to the net force and inversely proportional to the mass of the object. What this means is that when the force increases, so does the acceleration, which is a change in an object's state of motion. But as the mass increases, the acceleration decreases. Since the forces are equal, you can see that mass is really the key player in how an object is affected by the forces acting on it.

Putting it all together, it makes sense that the cannon does not accelerate as much as the cannonball because the cannon is far more massive. More mass means less acceleration. The cannonball has much less mass, so it is very much affected by the force from the interaction and shoots out of the cannon and travels through the air.

This is true for any action-reaction pair of forces. Say you're driving down the highway and a bug smacks into your windshield. Your car hitting the bug is the action and the bug hitting your windshield is the reaction. While the bug, unfortunately, meets an untimely death, your windshield hardly notices the event. The force exerted on the bug is the same force exerted on the windshield, but the effect on the bug is far greater because it has such a small mass compared to your vehicle!

Action Equals Reaction

There's one more thing to keep in mind about action-reaction forces: action always equals reaction. What this means is that no matter how much force you try to exert on an object, action can only occur with as much magnitude as the reaction can return.

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