Work, Energy & Power

Energy is the ability to do work Work is done when a force moves along its line of action When work is done energy is transferred from one type into another Energy and work are scalars and are measured in Joules

Work is defined as: Work Done = Force x distance moved in the direction of the force. W = F s

The joule is defined as: 1 Joule of work is done when a force of 1 newton moves a distance of 1 metre. 1 J = 1 Nm

Examples 1 How much work is done when a book of mass 2 kg is lifted 1.8 m from the floor to a shelf ?

2 The toy below is pulled a horizontal distance of 265 cm 2 The toy below is pulled a horizontal distance of 265 cm. Find the work done by the child.

3 A force of 36 N acts at an angle or 55o to the vertical 3 A force of 36 N acts at an angle or 55o to the vertical. The force moves its point of application by 64cm in the direction of the force. Calculate a) the work done by the horizontal component of the force, b) the work done by the vertical component of the force.

4 An elastic band is stretched so that its length increases by 2. 4 cm 4 An elastic band is stretched so that its length increases by 2.4 cm. The force required to do so increases linearly from 6.3N to 9.5N. Calculate: a) the average force required to stretch the elastic band, b) the work done in stretching the band.

5 A car weighs 10000 N. It travels 1 km along a sloping road at steady speed, ending up 100m higher than its starting position. Its engines provide a motive force of 4 kN. a) How much work is done by the engine? b) How much work is done against gravity? c) What other force is the engine working against? What is its value?

Example A log flume ride has a total mass of 1000 kg and is lifted 25 m above the ground. It is allowed to slide down a ramp of length 50m, obtaining a final speed of 15ms-1 at the bottom. Calculate: The GPE at the top The KE at the bottom The average frictional force.

Example A skier of mass 80 kg moves down a slope passing point A at 10 ms-1 and another point B at unknown speed. Point B is 100 m further down the slope and 40m below it. If the frictional force is 50 N, calculate the speed at B.

Example - challenging a) The work done by the winch. A winch pulls a log 50 m from rest along a rough plane inclined at 10 degrees to the horizontal against a frictional force of 200 N. The log has a mass of 500 kg and the winch pulls with a force of 1200 N. Calculate: a) The work done by the winch. The kinetic energy gained by the log. The speed obtained by the log at the end.