Work Done (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Work Done
Work is done when a force causes an object to move through a distance
If a force is applied but the object doesn't move then no work is done
When work is done energy is transferred
For example:
When work is done against frictional forces, energy is transferred by heating
When work is done against a gravitational field, energy is transferred mechanically
Work done to move a box
The formula for work done is:
Work = force × distance moved in the direction of the force
Where:
W = work done in joules (J) or newton-metres (N m)
F = force in newtons (N)
d = distance in metres (m)
Worked Example
A car moving at speed begins to apply the brakes. The brakes of the car apply a force of 500 N which brings it to a stop after 23 m.
Calculate the work done by the brakes in stopping the car.
Answer:
Step 1: List the known quantities
Distance, d = 23 m
Force, F = 500 N
Step 2: Write out the equation relating work, force and distance
W = F × d
Step 3: Calculate the work done on the car by the brakes
W = 500 × 23
W = 11 500 J
Worked Example
A meteorite travels at a constant speed through the vacuum of space. When it reaches Earth's atmosphere it burns up.
Describe the energy transfers taking place that cause the meteorite to burn as it enters the Earth's atmosphere.
Answer:
Step 1: Identify the initial store of energy
The meteorite is moving through space, therefore it has energy in its kinetic store
Step 2: Identify the energy transfer pathway
When the meteorite enters Earth's atmosphere, it experiences frictional forces due to air resistance, therefore energy is transferred by heating
Step 3: Describe the energy transfer
Energy is transferred from the kinetic store of the meteorite to the thermal store of the meteorite and the thermal store of the air particles
The energy transferred by heating is great enough for the meteorite to burn up
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