Okay, put away your books and take out something to think with. It's time for the UMOSPQ, the Unexpected Moment of Science Physics Quiz.
You have a long metal tube that is curved. You also have a miniature golf ball that you are knocking into one end and out the other. How does the ball move after it leaves the other end of the tube? Does it move in a straight line or a curve? Perhaps it starts by moving in a curved line, then gradually straightens out? Or something else?
When this question was given informally to a group of undergraduate students, about half of them suspected some kind of curved motion would remain in the ball after it leaves the tube. This, however, is incorrect. Once the ball leaves the tube it will move in a perfectly straight line. What's interesting, though, is that it wasn't until the 17th century that this was realized to be the case.
For hundreds of years people thought that some aspect of the thing that moved an object stayed in the object. That sounds a little weird, but if you thought the ball would continue in a curved path, you were subscribing to this theory. Curved tube, curved motion.
Newton's laws of motion stated that an object will always move in a straight line unless it is prevented from doing so. The curved tube prevents the ball from moving straight only as long as the ball is inside. Once free, however, it moves straight once again. And it will continue to do so as long as nothing interrupts this motion.
Newton always won at miniature golf.