Galilean Force Reasoning
Aristotle started our thinking about the relationship between force and motion. Heck, most people still think like Aristotle! Consider a block of wood on the ground. When you push it, it moves. When you stop pushing, it stops.
Aristotle concluded that you need a force in order to move. In other words, the natural state of an object is to be at rest. This can’t be the whole story though, because we know that motion is relative. Enter Galileo.
Galileo spent a long time considering the motion of wooden balls in wooden tracks. He noticed that a ball released from rest nearly returned to its same level. The more highly polished the ball was, the more nearly it returned to its original height.
He imagined that a perfectly spherical ball on a perfectly smooth track would return to its original level. He saw friction as a force that got in the way of an object’s natural tendency to keep going. So let’s imagine a situation with neither friction nor air resistance! Now imagine stretching the right hand side of the track out: note how the ball goes further each time.
Galileo asked what would happen if the right side of the track remained flat, that is, it never allowed the ball to return to its original level. What would the ball continue to do?
Galileo concluded that the natural tendency of an object is to maintain its velocity. In other words, you need a force to accelerate. Newton came along and made this his 1st Law: a = 0 if and only if Fnet = 0, where Fnet is the sum of the forces acting on an object. Makin’ sense?