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PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure

Physics 6.7 Friction — Useful and Unwanted Friction and Lubrication

5 min read 5 parts Lesson 7 of 19

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Physics 6.7 Friction — Useful and Unwanted Friction and Lubrication — Alhussan International Academy — Physics (Stages 7–9) notes: Describe the effect of friction on moving objects, how to reduce it and how it can be useful.Friction = a force between two surfaces in contact that …

  • Uneven surfaces
  • Starting motion
  • Ball bearings
  • Smooth and streamlined surfaces
  • Ice skates
  • Vehicle grip

Part 1 of 5What causes friction

  • Friction = a force between two surfaces in contact that opposes motion or attempted motion.
  • Uneven surfaces — All surfaces, even smooth-looking ones such as shiny metal, are uneven when seen under a microscope.
    • When surfaces slide over each other the bumps catch on one another.
    • This produces the force of friction that must be overcome.
  • Direction — Friction acts in the opposite direction to the motion.
  • Starting motion — If you push a book and it does not move, your force is balanced by friction.
    • When your force is bigger than friction, the forces are unbalanced and the book accelerates.
  • Energy — Friction transfers kinetic energy to thermal energy, so surfaces heat up and wear.

Part 2 of 5Reducing friction

  • Lubrication — A layer of oil between surfaces makes them slide over each other more easily and reduces friction.
    • Oil is used between moving engine parts and on bicycle chains.
  • Ball bearings — They roll over each other as a wheel turns, replacing sliding with rolling and reducing friction and wear.
  • Waxing — Skiers and snowboarders wax their skis and boards to go faster.
  • Smooth and streamlined surfaces — Polished surfaces and streamlined shapes reduce friction and drag.
  • Ice skates — The blade creates a thin layer of water on the ice that acts as a lubricant.

Part 3 of 5Useful friction

1Walking
— Friction between feet and ground makes it possible to move without slipping.
2Vehicle grip1 point
— Friction between tyres and road makes vehicles move.
  • In icy conditions friction is reduced and wheels skid because they cannot grip.
3Braking1 point
— Friction between brake blocks or pads and the wheels slows bicycles and cars.
  • The friction wears the brakes away, so they must be replaced.
4Rough surfaces
— Tyre treads and shoe soles are rough to increase friction.

Part 4 of 5Comparing useful and unwanted friction

Useful frictionUnwanted friction
Walking and grippingWear in machine parts
BrakingHeating of moving parts
Rough tyres and solesSlowing of sliding objects
  • Factors — Rough surfaces such as sandpaper give more friction than smooth ones such as glass, and pressing surfaces together harder increases friction.

Part 5 of 5Measuring friction

  1. Attach a newtonmeter to the object, such as a box on a surface.
  2. Pull the object steadily along the surface.
  3. Read the newtonmeter when the object moves at a steady speed.
  4. At a steady speed the pulling force equals the force of friction.

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