Forces, Motion & Friction

A force is a push or a pull. Forces start motion, stop motion, speed objects up, slow them down, or change their direction. Every time something moves, a force is at work — even when you cannot see what is causing it.

Pushes and Pulls

• Push — a force that moves an object away from the source of the force.
• Pull — a force that moves an object toward the source of the force.
• Forces have a size (how strong) and a direction (which way).
• Bigger forces cause bigger changes in motion.

What Friction Does

Friction is a force that happens when two surfaces rub against each other. It always works against motion — it slows moving objects down and can stop them altogether. Friction also turns some of the motion energy into heat, which is why your hands warm up when you rub them together.

• Rough surfaces have more friction than smooth surfaces.
• Brake pads use friction to slow a bicycle or car.
• Without friction, your shoes would slip and you could not walk.
• Oil and water reduce friction so parts can slide easily.

Everyday Examples

• Kicking a ball forward is a push.
• Opening a drawer toward you is a pull.
• Sliding on a carpet builds friction quickly.
• A parachute uses air resistance (a kind of friction) to fall slowly.