In this topic, you will learn about how forces affect the motion of an object.

When an object that was at rest starts to move, a change in motion occurs. A change in motion also occurs when a moving object speeds up, slows down, changes direction, or stops. A change in an object's motion is the result of all the forces acting on the object.

Think of what happens when you get off a seesaw. The forces suddenly become unbalanced, and the seesaw moves.

Unbalanced forces also create a change of motion in a tug-of-war. When both sides pull equally, the forces balance, and nothing moves. If one side begins to pull harder, the forces become unbalanced, resulting in a change in the rope's position.

Think about a ball rolling on the floor. Over time, the ball slows down, so a force must be acting on the ball. The force that slows down the ball is friction. Friction is a force that occurs when one object rubs against another object. In this case, friction occurs when the ball rubs on the floor.

Different materials produce different amounts of friction. Rough materials produce a lot of friction. Most smooth materials produce less friction. However, other materials, such as rubber, are smooth but still produce a lot of friction.

When you ride a bike, friction against the tires slows the bike down even if you are riding on a smooth sidewalk. You have to keep pedaling to keep the bike moving. How do brakes stop a bike's motion? The brake pad squeezes against the rim of the wheel, causing friction. The wheel slows down, and eventually, the bike stops.