6.2 Friction

What is friction?What is friction? Friction acts on an object which slides

or tends to slide over another object.

e.g. The car cannot travel without friction.

What is the effect of friction on our lives?

6.2 Friction

How can we measure the friction between a sliding

block and a table?

Pull the block with a spring balance

6.2 Friction

When the pulling force is small,pulling force = frictionThe block does not move.

pulling force

friction

6.2 Friction

Increase the pulling force such that

pulling forcemaximum friction between the block and the table=

The block still does not move.

pulling force

friction

6.2 Friction

pulling force

friction

Increase the pulling force such thatpulling force

maximum friction between the block and the table>

The block moves.Simulation: Friction

6.2 Friction

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.

6.2 Friction

Special carved patterns help to increase friction.

The grooved pattern underneath a hiking shoe

6.2 Friction

Rolling motion in ball bearings

Reducing frictionReducing friction

Lubricants used in bicycle gears

Can you give daily examples of reducing

friction?

6.2 Friction

A levitated object experiences negligible friction during motion.

An ‘air cushion ball’ game A Maglev

6.2 Friction

Fluid frictionFluid 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?

6.2 Friction

Airplanes are in streamlined shapes to minimize air resistance.

Swimmer wear smooth suit to reduce water resistance.

6.2 Friction

weight

Terminal velocityTerminal velocityWhen a skydiver just jumps out of an aircraft …

velocity = 0 force acting on him:

weight net force = weight acceleration = g

(downwards)

EXTENSION

6.2 Friction

When he moves faster...

weight

air resistance force acting on him:weight

( , < weight)

net force = weight – air resistance acceleration

( )

velocity

(constant)air resistance

6.2 Friction

air resistance = weight

Air resistance continues to increase such that

weight

air resistance

acceleration = 0 he moves at a

constant velocity

= weight – air resistance= 0

Terminal velocity

net force

Simulation: Skydiver

6.2 Friction

Terminal velocity depends on the(1) weight of the falling object

heavy objectlarge gravitational force

large terminal velocity

object with large surface area

large air resistance

small terminal velocity

(2) surface area of the falling object.

6.2 Friction

A parachute has a large surface area

Why can a parachute make the skydiver land

safely?

Terminal velocity of the skydiver

6.2 Friction

That’s the end of Section 6.2

Check Point

Key Ideas

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