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| Newton |
Newton's First Law: Unless an external force is applied, an object at rest will remain at rest, and an object moving in a straight line with constant velocity will continue to move in a straight line with constant velocity.
We don't have a problem with the first part of this formula, we see it all the time in everyday life - if a stationary object is not pushed, it stays still, does not move on its own. But we may have a little trouble understanding the latter part from everyday experience, because we do not see anything in motion going on forever. But the answer to this problem is given at the beginning of Newton's first law, which speaks of 'external force'. Whenever you set an object in motion, forces such as friction or air resistance act in the opposite direction to slow down the motion. Since there is no air in space, there is no air friction, so if an object could be pushed out there, it would continue at the same speed forever.
