1. What is the effect of friction on our lives?
What is friction?
Friction acts on an object which slides or tends to slide over another object.
What is the effect of friction on our lives?
e.g. The car cannot travel without friction.

2. How can we measure the friction between a sliding block and a table?
Pull the block with a spring balance

3. When the pulling force is small,
friction
When the pulling force is small,
pulling force = friction
The block does not move.

4. Increase the pulling force such that
friction
Increase the pulling force such that
pulling force
maximum friction between the block and the table =
The block still does not move.

5. Increase the pulling force such that
friction
Increase the pulling force such that
pulling force
maximum friction between the block and the table
>
The block moves.
Simulation: Friction

6. Friction depends on the nature of the sliding surfaces.
Rubber is used in tyres and brake pad on a bicycle
tyre
brake pad 
It can exert a large friction.

7. Special carved patterns help to increase friction.
The grooved pattern underneath a hiking shoe

8. Reducing friction Can you give daily examples of reducing friction?
Lubricants used in bicycle gears
Rolling motion in ball bearings

9. A levitated object experiences negligible friction during motion.
An ‘air cushion ball’ game
A Maglev

10. Can you give daily examples of reducing fluid friction?
Fluid friction – resistive force exerted on a body moving in a gas or a liquid.
e.g. air resistance, water resistance
speed of the object
fluid friction
Can you give daily examples of reducing fluid friction?

11. Airplanes are in streamlined shapes to minimize air resistance.
Swimmer wear smooth suit to reduce water resistance.

12. Terminal velocity When a skydiver just jumps out of an aircraft …
EXTENSION
When a skydiver just jumps out of an aircraft …
velocity = 0
force acting on him: weight
net force = weight
acceleration = g (downwards)
weight

13. net force = weight – air resistance ( )
When he moves faster...
velocity
force acting on him:
weight
weight
air resistance
(constant)
air resistance
( , < weight)
net force = weight – air resistance
( )
acceleration

14. Air resistance continues to increase such that
weight
air resistance
air resistance = weight
net force
= weight – air resistance
= 0
acceleration = 0
he moves at a constant velocity
Terminal velocity
Simulation: Skydiver

15. Terminal velocity depends on the (1) weight of the falling object
large gravitational force
large terminal velocity
heavy object
(2) surface area of the falling object.
object with large surface area
small terminal velocity
large air resistance

16. Why can a parachute make the skydiver land safely?
A parachute has a large surface area
Terminal velocity of the skydiver

17. That’s the end of Section 6.2
Check Point
Key Ideas
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Section 6.3
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