Work

1. 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.

2. 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.

3. Work done W = F x d F = 45 kN = 4.5x104 N, d = 20 m W = 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.

4. 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

5. 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

6. Change in height ( metres, m ) Potential Energy Potential Energy mass gravitational field strength ( joules, J ) (kilograms ,kg ) ( Newtons per kilogram, N kg-1 ) Energy stored in an object as it is lifted of the ground.

7. 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

8. Kinetic Energy Kinetic energy mass speed squared ( joules, J ) ( kilograms, kg ) ( metres per second squared Moving objects have kinetic energy, Ek,

9. 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 = ?