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

When an object that is resting starts to move, there is a change in motion. 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 that are acting on the object.

Think of what happens when you get off a seesaw. The forces suddenly become unbalanced. Then the seesaw moves. Unbalanced forces also create a change of motion in a tug-of-war. When both sides pull equally, forces balance. Nothing moves. If one side begins to pull harder, the forces become unbalanced. Then there is a change in the position of the rope.

Think about a ball rolling on the floor. Over time, it slows down. 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. The ball rubbing on the floor creates friction.

Different materials produce different amounts of friction. Rough materials rub best. They produce a lot of friction. Most smooth materials do not rub well. They produce less friction. Other materials, like 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 in order to keep the bike in motion. How do brakes stop a bike's motion? The brake pad squeezes against the rim of the wheel. There is friction between the brake pad and the rim. The wheel slows down. The bike stops.