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.
When you smashed into the bug, your windshield could exert no more force on the bug than the bug could exert on your vehicle. The same is true for the football - you can't kick it harder than it can kick you back.
Still don't believe me? Try this at home: hold a piece of paper in the air, drop it, and as it falls try to hit it as hard as you can. What you'll find is that no matter how hard you swing at it, you can't hit the paper any harder than it can hit you back. The paper simply isn't capable of exerting a very large amount of force, and since action and reaction are always equal in magnitude, this prevents you from exerting a large amount of action force on it.
Newton's third law of motion tells us that for every action force there is an opposed and equal reaction force. In any interaction between two objects, the first object exerts a force on the second, and the second object exerts a force back on the first object that is equal in magnitude and opposite in direction.
For simplicity, it's easiest to call the force from the first object the action, and the force from the second the reaction, but it really doesn't matter as long as you understand the interaction and how the objects are involved with each other.
Action and reaction forces don't cancel each other out because they act on separate objects. What's different in the interaction is the effect of the forces, not the forces themselves. Less massive objects will feel the effects more than objects that are more massive. This is because mass is inversely proportional to acceleration - the change in an object's state of motion.
More mass means less acceleration, less mass means greater acceleration for the object. An unlucky bug splatters on your windshield, because the acceleration for the bug is far greater than what your vehicle experiences!
Action and reaction forces are always equal in magnitude, so it's not possible to exert more force on an object than it can exert back. No matter how hard you try, you can't hit a piece of paper in the air harder than it can hit you back. Likewise, your windshield doesn't crack when the bug hits it because the amount of action force can only be as great as what that tiny insect can exert back in reaction.
Completing this lesson could lead to your ability to:
- Comprehend Newton's second and third laws of motion
- Point out the correlation between action and reaction and force
- Look at the relationship between mass and acceleration