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Me and some friends were discussing the idea of (in a perfect scenario, no air resistance or gravity to affect the results) 2 bodies moving in the same direction exactly parallel to each other.
If something was thrown from one of the objects directly towards the other so that it thrown at a right angle to the direction they are both travelling, would the item thrown hit the other body in the same place it was thrown from, but on the other body? The idea is that it would keep its vector going forward, and simply gain the vector going sideways, leading it to hit. However, some say that it would lose the forward vector when propelled and just miss the other body (assuming it's far enough apart that it wouldn't hit anyway). We can all see both sides of why it would and wouldn't work, but we can't decide what would actually work.
Sorry if this is a simple or silly question, I've never asked anything here before, it looks scary :)

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up vote 8 down vote accepted

The projectile thrown from one of the bodies would hit the other body. It is easiest to see this by considering the entire process in the frame of reference moving together with the two bodies, i.e. in their rest frame. In this frame, both bodies remain at the same position, the projectile is thrown and moves in a straight line towards the other body until it collides with it. The straight line movement is implied by the Newton's first law of motion since after the projectile is thrown no external forces act on it until the collision.

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