Friction Force
Senior 2 · Physics
Friction Force
Lesson 1 of 4
18 min
Let's begin
Rub your palms together quickly and they get warm. Try to walk on a freshly mopped classroom floor and your feet slip. A moto coming down a steep Kigali hill needs its brakes the whole way. All three involve the same force: friction. Without it you could not walk, write or hold a pen, yet it also wears out tyres and wastes fuel. This unit shows you how to use it and how to fight it.
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.
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.
Tip
Friction points "against the slide"
To find its direction, ask which way the surfaces would slip if there were no friction. Friction points the other way. Walking is the classic exam trap: friction on your foot acts forwards.
A book rests on a sloping desk without sliding. Which way does friction act on the book?
Solution: Without friction, the book would slide down the slope, so friction acts up the slope, along the desk surface.
A jerrycan of water is dragged at a steady speed along the ground with a horizontal pull of 80 N. What is the friction force?
Solution: At steady speed the forces balance, so friction = 80 N, opposite to the pull.
Why do the brakes of a moto become hot after a long ride down a hill?
Solution: The brake pads rub against the moving drum or disc. Work done against friction turns the moto's kinetic energy into thermal energy (heat) in the brakes.
Name two effects of friction you could observe on an old bicycle.
Solution: For example: worn tyre tread and brake blocks (wear), a squeaking chain (noise), or a chain that feels warm after a long ride (heat).
Summary
Sources and credits
The Rwandan examples, worked examples and practice items are written by Ganzaa.
