NASA - National Aeronautics and Space Administration

A force is something that changes the motion or the shape of something else. For example, when you push a cart, you put force on the cart to make it move. When you squeeze a piece of soft clay, the force you put on it changes the shape of the clay.

Image to right: When you put force on a cart by pushing it, the cart moves. Credit: World Book illustration by Lydia Halverson

Many forces affect the speed of a moving object. When you roll a ball across a rug, the friction, or rubbing, between the ball and the rug acts against the movement of the ball. So it slows the ball down.

Mechanical forces act when objects touch each other. Your body uses mechanical force when you pedal a bicycle or kick a ball.

Other kinds of forces act without touching objects. Instead, they act from a distance and cannot be seen. Gravity is this kind of force. It pulls everything toward the center of the Earth.

Image to left: When you push down on a pedal to make a bicycle move, you are using a mechanical force. Credit: World Book illustration by Lydia Halverson

Sometimes more than one force acts on an object at the same time. When two people push a stalled car, they use mechanical force to overcome the friction between the road and the tire. So the car moves forward. When you sit in a chair, gravity pulls you toward the Earth. But the chair pushes you up, away from the Earth. The two forces "cancel" each other out, and you stay still.

How to cite this article: To cite this article, World Book recommends the following format: "Force." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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