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Q & A: inertia

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Most recent answer: 08/27/2008
Q:
From inside a bullet train moving at 400 miles/hour, if we drop a ball inside a vacuum container, would the ball move backward at 400 miles/hour since there is no air in the vacuum container to hold the ball forward?
- Terry Hung 55
Fremont, CA, USA
A:
Absolutely not. All motion is relative. There is no reason at all for the ball to change its forward velocity (which is zero from the point of view of the train). No 'push' is needed to maintain motion, since from one point of view that condition isn't even moving.

This fact, first expressed clearly by Galileo and Descartes,  is the starting point of Newtonian physics. It's sometimes called 'the principle of inertia' or 'Galilean relativity.' It's also maintained in modern relativity.

Mike W.

(published on 08/27/2008)

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