Physics Asked by Jordan G on December 25, 2020
If I am a passenger who plays catching-the-ball game inside a vehicle that moves with a constant velocity in a straight road, why can I catch the ball repeatedly that as if the vehicle is at rest? How to explain this using first law of motion by Newton?
The explanation from the point of view of a stationary observer outside of the vehicle is that the ball has a certain horizontal velocity, which will remain constant unless it is acted on by a horizontal force (this is a consequence of Newton’s first law). If the ball is thrown straight up there is no horizontal force acting on it, so it continues to move with the same horizontal velocity. The thrower also has an identical horizontal velocity, so the ball remains directly above the thrower throughout it’s motion, and the thrower can catch it again as it falls.
From the point of view of an observer in the vehicle, there are no horizontal forces acting on either the thrower or the ball, so it is not surprising that the thrower can throw the ball vertically upwards and catch it again.
If your question is “why is Newton’s first law true” then the only answer is “that’s just the way our universe works”.
Correct answer by gandalf61 on December 25, 2020
That "constant velocity motion" is equivalent to "rest" is a very fundamental (and more deeper and mysterious, in my view) assumption about universe. So far no experiment is known that can distinguish the two. Rest and constant velocity motion just depend on choice of reference frame which is arbitrary.
This equivalence principle is more fundamental than Newton laws (in-fact it caused the Newton laws to be rewritten by Einstein). So you should derive Newton 1st law from this principle and not the other way around.
Answered by aman_cc on December 25, 2020
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