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Work & Energy (Edexcel GCSE Physics)
Revision Note
Work Done
- Work is done when an object is moved over a distance by a force applied in the direction of its displacement
- It is said that the force does work on the object
- If a force is applied to an object but doesn’t result in any movement, no work is done
Work is done when a force is used to move an object
Calculating Work Done
- The amount of work that is done is related to the size of the force and the distance moved by the object in the direction of the force
Work done = energy transferred
- To calculate the amount of work done or energy transferred for an object, the following formula is used
- Where:
- E = work done or energy transferred in joules (J)
- F = force in newtons (N)
- d = distance in metres (m)
- This equation can be rearranged with the help of a formula triangle
Work done, force, distance triangle
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.
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
Step 3: Calculate the work done on the car by the brakes
Examples of Work
- Work is done on a ball when it is lifted to a height
- The energy is transferred mechanically from the ball's kinetic energy store to its gravitational potential energy store
The weight on the ball produced by the gravitational field does work on the ball over a distance
- Work is done when a bird flies through the air
- The bird must travel against air resistance, therefore energy is transferred from the bird's kinetic store to its thermal store and dissipated to the thermal store of the surroundings
Air resistance (drag) does work against the bird as it flies through the air
Worked example
A woman draws a bucket up out of a well. The bucket has a mass of 12 kg when filled with water and the well is 15 m deep. Gravitational field strength is 10 N/kg.
a) Describe the energy transfer involved in raising the bucket out of the well
b) Calculate the work done on the bucket
Part (a)
-
- Energy is transferred mechanically (a force is acting over a distance)
- from the kinetic store of the woman (as she pulls the rope)
- to the gravitational potential store of the bucket (as it is lifted upwards)
- Part (b)
Step 1: List all of the known quantities
-
- Mass, m = 12 kg
- Gravitational field strength, g = 10 N/kg
- Height, h = 15 m
Step 2: Write the equation relating work, force and distance
Step 3: Write out the equation for weight and substitute it into the work equation
-
- Note: This is the equation for gravitational potential energy
Step 4: Calculate the work done on the bucket
-
- The bucket gained 1800 J of gravitational potential energy
Examiner Tip
Remember:
- Changes in speed are related to kinetic energy
- Changes in height are related to gravitational potential energy
- Changes in the shape of materials are related to elastic potential energy
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