What is friction? Name two factors it depends on. Give two advantages and two disadvantages of friction, and one way to reduce it.

1. 1 · Define. Friction is the force between surfaces in contact that opposes motion.
2. 2 · Say what it depends on. It depends on the roughness of the surfaces and on the normal reaction.
3. 3 · Two advantages.
   • We can walk without slipping.
   • Brakes can stop vehicles.
4. 4 · Two disadvantages.
   • It wastes energy as heat in machines.
   • It wears out moving parts and shoe soles.
5. 5 · One way to reduce it. Oil the moving parts, or use ball bearings to change sliding into rolling.
6. 6 · Close. So friction is useful, but it must be controlled.

Scoring tip: Write 'opposes motion' in the definition. Give the two factors by name: roughness and normal reaction.