Newton's first law of motion states that an object will keep a constant speed and direction unless acted upon by an unbalanced force. to test this statement, martin rolled a ball on a long, level street. the ball did not bump into any object, but it eventually came to a stop. how is this possible? martin did not roll the ball hard enough, which caused the ball to eventually stop rolling. the unbalanced force that caused the ball to stop was friction. every moving object must come to a stop because energy cannot be destroyed. newton's first law of motion only holds true in space, not on earth. martin initially applied a force to the ball, which started the ball moving. after the ball left his hand, the force he applied was no longer there.

so we have this ball rolling, but instead of keeping its constant speed it comes to a stop! and according to newton's first law, an object will keep a constant speed and direction unless acted upon by an unbalanced force. so if the ball comes to a stop, it has changed its speed, right? so there must be an unbalanced force!

our options:

1. martin did not roll the ball hard enough
well if he rolled it harder it would start out with a faster speed, but we know that it would eventually come to a stop.

2. the unbalanced force that caused the ball to stop was friction.
this makes sense. we know there is an unbalanced force on the ball, and friction happens to be a force that is always in the direction opposite of motion.

3. every moving object must come to a stop because energy cannot be destroyed.
energy cannot be destroyed, but it can be transferred! perhaps the energy is transferred out of the ball through friction ; )

4. newton's first law of motion only holds true in space, not on earth
newton's laws are universal! so you can apply them to earth, space, other planets, etc. he was actually one of the first ones to make the connection between motion on earth, and motion out in space. in the newton-apple story (when he sees the apple fall from the tree), he wonders if the force (gravity) that pulls the apple to the ground on earth could reach out all the way out to other planets!

i hope this gives you a little bit of insight.