What is friction?
Friction is a force that resists motion when two surfaces rub or slide against each other. It slows moving objects and produces heat.
Even surfaces that look smooth have tiny bumps and grooves. When two surfaces touch, these bumps catch on each other, and attractions between molecules also hold them together. Rougher surfaces and objects pressed together more firmly usually create more friction. Friction always acts in the direction opposite to motion.
There are several types. Static friction keeps a resting object from starting to move, such as a box on a ramp. Kinetic friction acts on objects already sliding. Rolling friction affects wheels and balls and is usually much smaller, which is why wheels make moving heavy loads easier. Air and water resistance are types of fluid friction called drag.
Helpful and harmful
Friction lets us walk without slipping, lets car tires grip the road and lets brakes stop vehicles. It also wears out parts and wastes energy as heat. Oil, grease and ball bearings reduce friction in machines. Rubbing your hands together to warm them uses friction too.
Did You Know?
Ice is slippery partly because a very thin layer at its surface behaves like a liquid, which reduces friction under skates and shoes.