-- If a moving object speeds up, slows, down, or changes its direction, then that has to be because there's some force on it.
-- If there's no force on an object, then it keeps moving forever, at a constant speed in a straight line.
-- We can never see that happening on Earth, because around here, there's always some force on a moving object ... like gravity, friction, and air resistance. In order for it to actually happen, it pretty much has to be happening in outer space.
Examples:
-- The Earth keeps moving around the sun, and TV satellites keep moving around the Earth, without slowing down.
-- Even the International Space Station, orbiting about 270 miles above the earth's surface, is almost outside all of the atmosphere, but not quite. There's still a tiny bit of air out there, slowing it down and 'decaying' its orbit. About once a month, they have to give it a short burst of rocket fire, to boost it back up to the altitude where it's supposed to be.
