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Slide 1

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Work, Energy and
Forces in Motion Topic 3…read more

Slide 2

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Work and Energy
· When you transfer energy you do work. We can measure the amount
of work done because:
Work Done = Force x Distance Travelled in the Direction of the Force
Joules = Newtons x Metres
e.g. Two farmers drag an old tractor tyre 5m over flat ground.
They pull with a total force of 340N.
What is the work done?
W = F x D 340 x 5 = 1700J.
FxD…read more

Slide 3

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Kinetic Energy
· Kinetic energy is energy of movement.
· It depends on its mass and speed.
Kinetic Energy = ½ x Mass x Velocity²
Joules = ½ x kg x m/s²
e.g. A 100kg bicycle moves at 8 m/s². Important to
Calculate its kinetic energy. square the
velocity first
½ x 100 x (8²) = 3200J/3.2kJ. K.E.
½ x M x V…read more

Slide 4

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Potential Energy
· Energy can be stored as potential energy.
Elastic Potential Energy
· Energy stored when work is done on an elastic object to change its shape.
· Released as kinetic energy (and some heat) when the object springs back
into shape.
Gravitational Potential Energy
· Energy stored in an object when it is risen against gravity.
· You have to move something to increase its gravitational potential energy
and that energy is only released when the object falls.
Gravitational Potential Energy = Weight x Height
Joules = Newtons x Metres
Weight = Mass x Gravity
WxH…read more

Slide 5

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· If one person does the same amount of work as another, but does it
more quickly, they are said to be more powerful.
Power = work done = energy transferred
time taken time taken
e.g. George pushes a shopping trolley with a force of 100N for 20m in
10 seconds.
Find the power.
Work done = 100 x 20 = 2000
Power = 2000 ÷ 10 = 200 Joules
PxT…read more

Slide 6

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Momentum = Mass x Velocity
kg.m/s = kg x m/s Mass x Velocity
· Momentum is conserved momentum before = momentum after
e.g. Two skaters approach each other, collide and move off together.
At what velocity do they move after the collision?
1. Explain which direction they move to to the right
2. Total momentum before collision = momentum of A + B
= (80 x 2) + (60 x -1.5) = 70 kg.m/s²
3. Total momentum after collision = momentum of A + B
= 140kg x V
Velocity (V) = ?
4. So 140 x V = 70kg V = 0.5 m/s to the right
1.5 m/s
2 m/s
80kg 60kg
(80+60) kg
Before After…read more

Slide 7

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