The Curiosity Shop

Action and Reaction - Newton's 3rd Law

The basic idea is that ALL forces either attract or repell two objects. The movement of the one object is the action and that of the other is the reaction. Which is which? It doesn't matter. Take your choice.

One of the common examples I have heard given for Newton's Third law is that of the force exerted by one train car on the next. It is said that one car pulls the one behind it, and that car pulls back with an equal force in the opposite direction. That may seem reasonable on the surface, but what if the train is accelerating? Think about the coupling between the cars. If the pulling forces were equal it would not accelerate with the rest of the train. The forces would cancel out and according to Newton's second law, "Force equals Mass times Acceleration". If the force is zero and the mass is not zero then the acceleration is zero.

Rail cars

So what's the problem here? Does Newton's Third Law only to apply to objects that are not acceleration? No. The problem is that the example has nothing to do with Newton's Third Law, unless you start looking at things on the atomic level. The train is, for the purpose of this discussion a rigid body. There are no parts of it that are being moved together or apart, so we are not going to see Newton's Third Law at work within the train itself.

Where will we see Newton's Third Law at work? Only where there are things being pushed apart or pulled together. We could look at the inner workings of the engine, but the more obvious separation is between the engine where its wheels push backward against the track. The train moves forward, that's an action. What's the reaction then? It is the earth, the whole planet that is, moving back. It doesn't move back much of course. The ratio of how much the earth moves compared to how much the train moves forward is the same as the ratio of how much the train weighs to how much the earth weighs, and that's not much. You can't see it. So maybe the train is a bad example, but it's a common one nevertheless.

What are some better examples? A jet engine is a good one. The engine body and the exhaust are pushed apart by the pressure in the engine. The exhaust goes backward and the engine goes forward. Firing a gun is another example where the reaction is big enough to notice. The explosion of the gunpowder pushes the bullet forward and pushes the gun back equally. The bullet goes flying because it is little and light weight. The gun and the person holding it are much heavier and friction where the person meets the ground tends to hold them in one place, but still the recoil is noticable.

How about things that are pulled together? Hold a magnet and a piece of iron suspended from threads. Bring them close and you will see that they move toward eachother. If they weigh the same the equal force on each, or as I'd prefer to say, the one force between them, moves them equally. Of course if the piece of iron out weighs the magnet by two to one you will see the force move the magnet twice as much as the piece of iron. If the magnet is small and the iron is a refrigerator sitting on the kitchen floor the magnet moves far more than the refrigerator and the earth to which friction attaches it. However, if you bring the magnet close and let it pull itself to the refrigerator-earth combination, the earth will react, but fortunately not enough to see.

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