54
The force of friction
Textbook: Part 2, pp. 90–93
GoalKnow friction, its benefits and harm, and ways to increase and reduce it.
New words
friction · ishqalanishsmooth · silliqrough · g‘adir-budurgrease (oil) · moy
Explanation
The force that appears when two bodies touch and slide, or try to slide, against each other and that hinders motion is called friction. The rougher the surface, the stronger the friction. Friction is useful: thanks to it we walk without slipping, car tyres grip the road, a match lights and brakes stop a car. On ice friction is small, so it is slippery; in winter sand is spread on roads. Friction can also be harmful: machine parts rub, heat up and wear away. To reduce it, parts are oiled, ball bearings are fitted and surfaces are polished. Rubbing hands to warm them is friction too.
Worked examples
A box is hard to slide on a thick carpet — friction is large; on ice it slides easily — small.
Class activity
Slide a book across a table, a cloth and paper by pulling with a string. On which surface is it hardest to move? Record it.
Practice
1
What is friction?
The force between touching, sliding bodies that hinders motion.
2
How can friction be reduced?
Oil it, polish the surface, fit ball bearings.
3
Why do we slip on ice?
Friction on ice is small.