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I tried to explain how those force work but I can hardly figure it out. I exerted a force on the trolley and there will be a force on trolley on me as well. This is the newton's third law. But why the trolley will move? Where's the force come from

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Possible duplicates: physics.stackexchange.com/q/45653/2451 and links therein. –  Qmechanic Jun 2 '13 at 8:34
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4 Answers 4

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The trolley moves because of the force you exert on it.

The reaction force applies to you, not to the trolley. It does not affect the trolley.

You feel the reaction force pressing the skin of your hands. You don't move backwards because your feet are also exerting a force on the ground and the reaction force pressing on your feet cancels out the reaction force pressing on your hands (they are opposite and approximately equal).

Since the planet weighs a little bit more than the trolley, it moves a little bit less. You probably didn't notice it.

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The trolley only moves because of forces acting on it. The reaction force acts on you, not the trolley. It is the force of you pushing on the trolley that makes it move. Learn about 'free body diagrams'.

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You are applying a force on a trolley that will move the trolley. The equal and opposite reaction force is the trolley applying a force on you. This reaction force is not immediately intuitive because you don't move -- because you're standing on the ground and your feet have a lot of friction.

Imagine what would happen if you didn't have a lot of friction. For example, if you were wearing roller skates, the equal and opposite reaction would be that you would start rolling away from the trolley (in addition to the trolley rolling away from you)

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The action reaction forces operate on different bodies. If you push a trolley, it pushes on you. The sum of the forces will still be zero, but it will be a different kind of zero, because the trolley will push you backwards. In a free body diagram of the trolley, it will have the force from your push as the net force in the x-direction, and you will have a net force in the direction opposite to the trolley. That is how the trolley will move.

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