Friction Force
Senior 2 · Physics
Friction Force
Lesson 3 of 4
20 min
Let's begin
A bicycle chain that squeaks is wasting your effort, and a few drops of oil fix it. A lorry's worn tyres on a wet road cannot grip, and new treads fix that. Controlling friction, sometimes reducing it and sometimes increasing it, is a large part of mechanical engineering and of every mechanic's work in a Kigali garage.
Byiringiro drags a 25 kg sack of maize across a cement floor at steady speed. μk = 0.40 and g = 10 m/s².
Tip
"Oil, Roll, Smooth, Stream": lubricate, use rollers or bearings, polish, and streamline.
In an explanation, always state HOW the method works. For example: "ball bearings replace sliding friction with much smaller rolling friction".
Explain how oil reduces friction in a moto engine.
Solution: The oil forms a thin film between the moving metal parts, so their surface bumps do not touch and catch. The film is also easy to shear, so there is much less friction and less heating and wear.
A crate needs 120 N to slide at steady speed. What force is needed if a second, identical crate is stacked on top?
Solution: The normal reaction doubles, so friction doubles: 240 N.
Why do new tyres have deep grooves (treads)?
Solution: On a wet road the grooves carry water away from under the tyre, so the rubber stays in contact with the road and keeps its grip (friction).
Give one way of reducing drag on a cyclist.
Solution: Crouch low to reduce the area facing the air, or wear smooth, tight clothing (streamlining).
Summary
Sources and credits
The Rwandan examples, worked examples and practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
