Question

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.

Answer 1

I think the correct answer from the choices listed above is option 2. The ball stop because there is an additional force that is present that causes it to be unbalanced. 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. Hope this answers the question. Have a nice day.

Answer 2

Answer:

The unbalanced force that caused the ball to stop was friction

Explanation:

As Newton's second law states, the acceleration of an object is proportional to the net force applied on the object:

$F=ma$

therefore, in order to move at constant speed, an object should have a net force of zero (balanced forces) acting on it.

In this case, the ball slows down and eventually comes to a stop: it means that the ball is decelerating, so there are unbalanced forces (net force different from zero) acting on it. The unbalanced force acting on the ball is the friction: friction is a force against the motion of the object, which is due to the contact between the surface of the ball and the surface of the street, and this force is responsible for slowing down the ball.