You have probably heard the word "force" before in conversations. Here are a few examples: "the rocket had a lot of force at blast off" or "the force of the storm blew the roof off the building." What is force? Force is defined as a push or pull on an object.

When you ride a bike, your foot pushes against the pedal. The push makes the wheels of the bike move.

When an apple falls from a tree, it is pulled to the ground by gravity.

Forces affect how objects move. They may cause motion; they may also slow, stop, or change the direction of motion of an object that is already moving.

Since force cause changes in the speed or direction of an object, we can say that forces cause changes in velocity. Remember that acceleration is a change in velocity. So forces cause acceleration.

Remember the following things about forces:

• Forces are measured in Newtons (N).
• Forces act in pairs.
• Forces act in a particular direction.
• Forces usually cannot be seen, but their effects can.

Sources of images used for this section as they appear, top to bottom:

• Transportation-Bicycle, Animated Gifs
• Apple Falling, NASA