Equal and Unequal Forces

By the end of this lesson, you will be able to explain how equal and unequal forces change the way an object moves.

A force is a push or a pull on an object. You use forces all day long. You push a door, pull a wagon, or kick a soccer ball. Some forces are strong, and some are gentle. When forces act on an object, they decide if it stays still, speeds up, slows down, or changes direction. That pattern of movement is called motion.

Think about a tug-of-war game at field day. Two teams pull on the same rope in opposite directions. If both teams pull with the same strength, the forces are equal. The rope does not move. Equal forces that push or pull in opposite directions cancel each other out, so the object stays where it is.

Now imagine one team gets tired and the other team pulls harder. The forces are now unequal. The rope moves toward the stronger team. Unequal forces always cause a change in motion. The object starts moving, stops moving, speeds up, slows down, or turns a different way.

Forces also move energy from one place to another. Energy is the ability to make something move or change. When you kick a soccer ball, energy from your moving leg transfers to the ball, and the ball zooms across the grass. When the ball rolls into a friend's shoe, energy transfers again and the friend's shoe pushes it a new direction. The harder the kick, the more energy transfers, and the farther the ball goes.

So watch for patterns. Equal forces keep things steady. Unequal forces cause change, and energy moves along with that change.

Try it: If you push a book across your desk and it slides, then stops, were the forces on the book equal or unequal when it stopped?