Forces and Motion

A force is a push or a pull. Every time you kick a ball, open a door, or stretch an elastic band, you are using a force. Forces can make things start moving, stop moving, speed up, slow down, or change direction. Forces are happening all around you, all the time.

When you slide your hand along a rough surface, a force called friction tries to slow the movement down. Friction happens whenever two surfaces rub together. It is why your shoes grip the floor when you walk, why a bike slows down when you brake, and why rubbing your hands together makes them warm.

Friction is not always helpful, though. It can wear things out and waste energy. That is why we add oil to bike chains and machines — to make surfaces slippery and reduce friction so they move more smoothly.

Forces are measured in a unit called the Newton (N), named after the scientist Sir Isaac Newton. A spring balance, also called a force meter, is the tool scientists use to measure how big a force is. The bigger the force, the more Newtons it measures.

Often more than one force acts on something at the same time. When the forces are equal and pulling in opposite directions, they are balanced, and the object stays still or keeps moving steadily. When the forces are unequal, they are unbalanced, and that is when motion changes — the object speeds up, slows down, or changes direction.