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1.7.5Motion, Forces and Energy

Work Done — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 80 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.7.5 Work Done — IGCSE PHYSICS notes: Define work done as energy transferred by a force and calculate it using W = Fd.Work done = the energy transferred when a force moves an object in the direction of the force.Work done and energy transferred are equivalent, so W = ΔE.Examples…

  • No movement means no work
  • What the symbols mean
  • Rearranged forms are F
  • Force at right angles to the motion
  • Lifting at steady speed
  • Accelerating an object

Part 1 of 7Work and energy transfer

Work and energy transfer
Work done
Work done the energy transferred when a force moves an object in the direction of the force.
Work done and energy transferred are equivalent, so W
ΔE.
Examples
  • Examples lifting a box, pushing a car, a motor winding a rope and friction slowing a bicycle.
No movement means no work
  • No movement means no work pushing against a solid wall transfers no energy to the wall.
Mechanical work is done by a force and electrical work is done by a charge moving through a potential difference.

Part 2 of 7The equation

1Equation
  • W = Fd.
    What the symbols mean: W = work done = energy transferred (J) · F = force (N) · d, s or x = distance moved in the direction of the force (m)
  • W is the work done in joules (J).
  • F is the force in newtons (N).
  • d is the distance moved in the direction of the force in metres (m).
2Joule
  • Joule the work done when a force of 1 N moves an object through 1 m in the direction of the force, so 1 N m = 1 J.
3Rearranged forms are F
W/d and d = W/F.
4Force at right angles to the motion
  • Force at right angles to the motion it does no work, as gravity on a satellite in circular orbit.

Part 3 of 7Where the energy goes

Where the energy goes
Lifting at steady speed
  • Lifting at steady speed the work against weight is stored as gravitational potential energy, W = mgΔh.
Accelerating an object
  • Accelerating an object the work done by the resultant force becomes kinetic energy.
Against friction
  • Against friction the work becomes thermal energy in the surfaces and surroundings.
  • A block dragged at constant speed has pull equal to friction, and all of F × d goes to the thermal store.
Stretching or compressing a spring
  • Stretching or compressing a spring the work is stored as elastic energy.
Braking
  • Braking the work done by the brakes equals the kinetic energy lost, F × d = ½mv².

Part 4 of 7Using work in calculations

Using work in calculations
Identify the force in the direction of the motion.
Convert the distance to metres and the force to newtons.
Multiply F × d and give the answer in joules.
Larger units
  • 1 kJ = 1000 J and 1 MJ = 1 000 000 J.
Rearranging
  • Rearranging a machine that does 600 J of work over 2.0 m exerts an average force of 300 N.
Friction means the work done by the person or engine is larger than the useful gain in energy.

Part 5 of 7Force–distance graphs

Force–distance graphs
When the force changes during the movement, the work is the area under the force–distance graph.
Constant force
  • Constant force the graph is a rectangle with area F × d.
Force increasing steadily from zero, as for a spring obeying Hooke's law
  • Force increasing steadily from zero, as for a spring obeying Hooke's law the graph is a triangle with area ½Fd.

Part 6 of 7Force, direction and work

Force, direction and work
Only the part of a force along the direction of motion does work.
Carrying a bag horizontally at constant height
  • Carrying a bag horizontally at constant height the upward force is at right angles to the motion, so no work is done on the bag.
Work against a force opposing motion removes energy from a moving object; friction removes kinetic energy and adds thermal energy.

Part 7 of 7More from the specification

1
Work Done as Energy Transfer
  • Mechanical work done by a force moving an object over a displacement is measured in joules (J) and represents energy transferred between energy stores (W = F · d).
2
Conservation of Energy in Systems
  • The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another within a closed physical system.
3
Efficiency and Dissipated Energy
  • Useful energy efficiency is calculated as × 100%; thermal heating due to friction is dissipated into surroundings as wasted energy.
    • (useful output / total input)
4
Power and Work Rate
  • Power is the rate at which energy is transferred or work is done per unit time (P = W/t), measured in watts (W), where 1 W = 1 J/s.
5
Efficiency
  • Useful output divided by total input.
6
Wasted energy
  • Energy transferred to stores that are not useful, usually thermal in the surroundings.
7
Energy store
  • Where energy is held, such as kinetic or chemical.
8
Gravitational field
  • Region where mass experiences gravitational force.
9
Transfer pathway
  • The way energy moves between stores, such as heating or work done.

Key terms

TermMeaning
Work doneEnergy transferred when a force moves an object; W = Fd.
JouleWork done by 1 N over 1 m in the direction of the force.
Force–distance graphGraph where the area under the line equals the work done.
Dissipated energyEnergy spread into the surroundings as thermal energy.
Elastic energyEnergy stored in a stretched or compressed object.

Avoid these mistakes

1
Using the total distance when the force acts for only part of the movement.
2
Using a force that is not in the direction of movement.
3
Saying work is done holding a heavy object still — with no movement no work is done.
4
Confusing work (J) with power (W).
5
Using cm or kN without converting to m and N.

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