Meaning of Friction and Its Effects

1What friction is

Friction is a force that opposes the relative motion, or the attempted motion, of two surfaces in contact. It acts along the surfaces (parallel to them) and in the direction opposite to the way they slide, or would slide, over each other.

2Why friction happens

Even surfaces that feel smooth are rough when magnified: they are covered in tiny bumps and hollows. When two surfaces touch, the bumps catch on each other, and where they touch closely the atoms of the two surfaces pull on each other. Before one surface can slide over the other, these contacts must be dragged over or broken. That resistance is friction.

Open the Friction simulation. Rub one book against another and zoom in on the atoms: they jiggle faster as the books get warmer, and some break off. That is heating and wear, seen at the level of atoms.

Worked example: which way does friction act?

• Pushing a crate to the right. The crate tends to slide right over the floor, so friction on the crate acts to the left.
• Constant speed. Mugisha pushes a crate with 50 N and it slides at a steady speed. Its velocity is not changing, so the forces on it are balanced. The friction must be 50 N, acting opposite to his push.
• Walking. Your foot pushes backwards on the ground. Friction stops your foot slipping backwards, so friction on your foot acts forwards, and that is the force that moves you forwards. On ice, with little friction, your foot slides back and you cannot walk.

3Effects of friction

• It opposes motion: moving objects slow down and stop unless something keeps pushing them.
• It produces heat: the work done against friction becomes thermal energy. Brake drums get hot, and a match lights when struck.
• It causes wear and tear: tyres go bald, shoe soles thin, brake pads wear out, and engine parts wear down.
• It causes noise: brakes squeal, and a dry door hinge creaks.
• It provides grip: it lets you walk, lets tyres push a vehicle forward, and holds a nail in wood.

Practice