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Newton's 3rd Law.


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We are told that "The action and reaction forces between a pair of interacting bodies are of equal magnitude and are opposite in direction"

 

So if a locomotive exerts a force of say 10000N on a wagon (via the coupling) we know that the wagon exerts exerts an equal force of 10000N on the locomotive in the opposite direction, why then does the train move? Doesn't each force cancel each the other out?

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The force keeps the locomotive and the wagon moving together. They don't move independently of one another. It has nothing to do with just the locomotive moving. That is just an internal force, the external force, like the force the engine applies on the tracks to make the train move is independent of the internal forces keeping the train together.

 

If you glue a paperclip to a baseball so that the coupling between the baseball and the paperclip is 10,000N, can you not still throw the baseball? The baseball still moves, because its movement is independent of how strong the glue is that is keeping the paperclip attached.

 

Even beyond that... Newton's Third Law doesn't prevent things from moving, either.

 

The train's engine pushes against the track. The track will push back against the train in an equal and opposite direction. This causes the train to move. The tracks are anchored to the ground. So, in reaction to the train pushing against the track, the track pushes against its anchors in the ground. The track pushes against the entire earth. And, the entire earth does move in response to this push -- it is just that the entire earth is so massive the movement caused is very, very tiny. Imperceptibly tiny.

 

Same thing when you jump. You push off from the earth. You move up and the earth moves down. It is just that the earth moves down just a minuscule amount. Minuscule enough that for all practical purposes it is zero. But it does move, and that movement is a direct consequence of Newton's Third Law.

Edited by Bignose
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If you glue a paperclip to a baseball so that the coupling between the baseball and the paperclip is 10,000N, can you not still throw the baseball? The baseball still moves, because its movement is independent of how strong the glue is that is keeping the paperclip attached.

 

Measure the speed you can throw that ball without the paperclip. Now glue the paperclip to the ball and retest the speed you can throw it.

 

The force you exert on the ball is less without the paperclip, and a greater acceleration occurs when thrown.

 

When the paperclip is glued to the ball the acceleration decreases, and the force increases.

 

You can throw a ball faster, not harder.

 

In order to throw it harder more mass needs to be added.

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We are told that "The action and reaction forces between a pair of interacting bodies are of equal magnitude and are opposite in direction"

 

So if a locomotive exerts a force of say 10000N on a wagon (via the coupling) we know that the wagon exerts exerts an equal force of 10000N on the locomotive in the opposite direction, why then does the train move? Doesn't each force cancel each the other out?

 

An object's motion is dictated by the forces acting on that object. As book worm has noted, action/reaction forces act on different objects. The don't cancel because they should never show up in the same equation of motion of an object.

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Thanks for your responses. So going all the way down the "pushing tree" a train moves because the loco pushes against Earth and because the Earth has a far greater mass than the loco the loco is propelled forward?

 

The train moves because the force (load) on the crank is less than required to stop the increase in rotational velocity of the crank at that specific RPM of the crank.

 

The more load you put on the crank the lower the acceleration rate of the crank, to the point where the load is so great on the crank that it can no longer accelerate.

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Thanks for your responses. So going all the way down the "pushing tree" a train moves because the loco pushes against Earth and because the Earth has a far greater mass than the loco the loco is propelled forward?

 

Yes. If you look at this from the proper reference frame (with a wide perspective), all of the forces are internal and momentum will be conserved. The train moves one direction and the earth will move in the other. However, that may not be the most convenient way of determining the motion.

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Measure the speed you can throw that ball without the paperclip. Now glue the paperclip to the ball and retest the speed you can throw it.

 

The force you exert on the ball is less without the paperclip, and a greater acceleration occurs when thrown.

 

When the paperclip is glued to the ball the acceleration decreases, and the force increases.

 

You can throw a ball faster, not harder.

 

In order to throw it harder more mass needs to be added.

 

I never said that there would be a difference between how far or fast or anything like that when you put the paperclip on the ball. The point is that the internal forces acting on the ball -- the glue between the paperclip and the ball -- do not change the ability for an outside force to act on the system. Namely a person throwing it.

 

The coupling between a locomotive and an attached car is similar. No matter how weak or strong the force attaching the two cars are, an outside force can still move them. That was my point. It has nothing to do with speed or acceleration. Just the fact that the internal forces really have nothing to do with the external forces.

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