Work

• Work done is a measure of the energy transferred. E.g. when lifting a pencil I do work against the earth’s gravity force, energy has been transferred: chemical energy in my body has been turned into kinetic energy which is turned into potential energy as the pencil gains height.

Work

Work done = Force x distance

W = F x d

Joules, J Newtons, N metres, m

Example

Calculate the work done by the brakes of a car if a 4.5kN average force are applied over a distance of 20 m.

Work done W = F x dF = 45 kN = 4.5x104 N, d = 20 mW = 4.5 x 104 x 20 W = 9.0 x105 J The car originally had 9.0 x105 J of kinetic energy. To bring the car to rest the brakes must do the exact same amount of work, 9.0 x 105 J.

Power• This is the rate of doing work ( transferring energy )• i.e. work done per second• Units are Watts, W• 1 W = 1 J s-1

• i.e. a 2000 W heater transfers 2000 J of electrical energy into heat energy every second

Power

Calculate the work done by a 75kW motor running for 5 mins.

P = 75 kW = 75 000 W

t = 5 mins = 5 x 60 = 300 s

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EP 71025.260575000

Potential Energy

Energy stored in an object as it is lifted of the ground.

hxgxmEp

Potential Energy mass gravitational field strength

( joules, J ) (kilograms ,kg ) ( Newtons per kilogram, N kg-1 )

Change in height

( metres, m )

Potential energy This is really a special case of ‘doing work’Work done = average force x distance moved

If we lift something vertically at uniform speed then the forces acting on it are balanced

Fup = Fgravity = m x g

For a vertical distance we normally write h instead of d Therefore W = f x d W = m x g x h

Kinetic EnergyMoving objects have kinetic energy, Ek,

25.0 vxmxEk

Kinetic energy mass speed squared

( joules, J ) ( kilograms, kg ) ( metres per second squared 21sm

Kinetic energy

Calculate the speed a 1000 kg van is moving at if it has 50 000 J of Ek.

Ek = 50 000 J, m = 1000 kg v = ?25.0 vxmxEk

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