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A rock is thrown towards a window, and hits it. The window does not break and the rock reverses its direction and falls under the window.

My question is, under this circumstance since the rock reversed it's direction of movement I want to say that the force exerted by the window was larger than that of the rock because if they were equal, the rock would simply stick to the window and slide down until it reaches the bottom.

However, the answer to this problem claims that the window and the rock had the same force. Can someone explain me what is going on?

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1 Answer 1

The window and the force have to have the same force but on each other. This is dictated by Newton's third Law of Motion. The rock exerted a force on the window which was not enough to break it. The window, in turn, exerted an equal and opposite force on the rock which was quite enough to make it reverse its direction of motion.
This is what is happening in this situation.

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I thought that if the net force equals zero, it will stop accelerating. So my question is, if the window exerts the same force as the rock does to the window, the rock should stop moving towards the window and stop, instead of heading the other direction. Does that make sense ? Or, is there another force that is acting on the rock that accelerates it in the opposite direction ? – hyg17 Dec 8 '13 at 0:45
This is a common misconception about Newton's Third Law of Motion. What I wrote in my answer means that the two equal and opposite forces act on different bodies, not the same one. The rock exerts force on window, in turn, window exerts force on rock but two forces do not act on the rock or the window. So, the rock wouldn't stop moving and will go in the opposite direction. – Ris97 Dec 8 '13 at 3:15

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