You have asked a great question, one that touches on one of the most common interactions between things in our world. It's the same question many great thinkers have asked for centuries.

If you ask any scientist on the street, "What makes a ball stop when it is rolling?" they'll give you at least one word: friction. Let me ask you, what do you think of when you hear the word friction? Maybe you think of two people arguing, or something sliding, slowing, and heating up, or just a general feeling of difficulty.

Friction in our world is a force. What is a force? A force causes acceleration, or a change in velocity. This change in velocity could be from rest to 60 miles per hour, or from 10 miles to per to 3 miles per hour. Say you push your friend. You pushing your friend is a force because you cause your friend to accelerate. Friction is a force that resists movement. It always opposes the motion of an object. Friction does not happen in and of itself, it needs at least two players that are touching one another. These players may be a ball and the ground, a book and a table, a skydiver and the air, or any number of objects that are made from that fundamental building block of nature--atoms.

When two objects are in contact and moving relative to one another, friction happens. When I say moving relative to one another, I mean one is moving faster or slower or in the opposite direction compared to the other. A book slides on a table, and friction exerts a force in the opposite direction the book is moving until the book stops. A skydiver falls through the air and speeds up until there are so many air molecules hitting him per second that they keep gravity from accelerating him anymore.

What is friction really? We said friction is a force. Well the specifics of that force cannot be found in our world, but the world of atoms. The truth is that friction is fundamentally electromagnetic in nature! Think about bringing two positive ends of a bar magnet together. They repel each other, right? The same thing is happening in a place smaller than one billionth of a yard. Everything in our world is pushing against each other.

When you set your keyboard on your desk, it doesn't fall through the desk because the electrons in the atoms on the surface of your desk are pushing against the electrons in the atoms on the bottom of your keyboard. When you roll a ball on the ground, the electrons in the atoms on the surface of the ground push against the electrons in the atoms on the surface of your ball that is touching the ground.

A rolling ball stops because the surface on which it rolls resists its motion. A rolling ball stops because of friction.

Thank you for the question. The answer to your question is friction. As you may know, friction is the resistance that one surface of an object encounters when another one touches it. Think about if you were to jump on a water slide, you would be able to go a long distance because there is less friction, but if you were to do that to a carpeted floor, you probably wouldn't get very far, that is because there is a lot of friction.

When the ball rolls across the floor, the floor opposes the motion of the ball which takes away from the momentum of the ball.