Friction

What is a friction?
Friction is the contact force that opposes or tends to oppose motion between surfaces in contact.

• Opposes relative motion
• Tendency towards motion
• Static friction > kinetic friction

Advantages of Friction:

• We can walk without slipping
• Moving vehicles are able to slow down when needed

Disadvantages of Friction:

• Cars are less efficient by up to 20%
• Moving parts in engines, motors and machines suffer wear and tear

Ways to reduce friction:

1. Lubricants / polished surfaces:
   Lubricants greatly reduce friction & wear & tear. Polishing a surface removes surface irregularities
2. A layer of air/air cushion:
   Ex. Air hockey
3. Ball bearings:
   Placed between moving parts, ball bearings roll around and prevent moving parts from rubbing against each other
4. Wheels:
   Being circular in shape, wheels greatly reduce the friction between two surfaces

Uses of friction:

• Treads (grooves in tyres that channel water out from underneath the tyres)
• Parachutes (skydiver varies his air resistance to change his speed)
• Chalk (absorb perspiration and improve grip)
• Walking without slipping
• Grip objects
• Start an object moving

Classic Questions of Friction

1. Suggest and explain why a car moves at constant velocity after some time even though the car engine continues providing the same amount of forward driving force (thrust).

• During acceleration, the car gains speed and the backward resistive forces (air resistance and contact friction with road) increases.
• It comes to a point where backward resistive forces equals the forward driving force (thrust)
• The resultant force acting on the object is now zero. Therefore the acceleration is zero and it continues moving with constant velocity (known as terminal velocity).