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2.1.1Energy

Work Done

5 min read 5 parts Lesson 2 of 16 1 quiz

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Section 1 of 5Work and Energy

2.1.1 Work Done

Work done is a transfer of energy, and it only happens when a force moves an object.

Work done
  • The energy transferred when a force moves an object through a distance in the direction of the force.
No movement
  • No work is done when a force is applied but the object does not move, because no energy is transferred by the force.
Work against friction
  • Work done against friction transfers energy to the thermal store, which heats the surfaces involved.
Work against gravity
  • Work done against weight transfers energy mechanically to the gravitational potential store of the object.

Section 2 of 5Calculating Work Done

Only the component of the force in the direction of motion does work, so the distance must be measured along the force.

1
Identify the force in newtons and the distance moved by the object in the direction of the force, in metres.
2
Check that the force and the displacement point in the same direction, because a force at right angles to motion does no work.
3
Use SI units throughout, so that a force in newtons and a distance in metres give the work done in joules.
4
Multiply the force by the distance using W = F × d to obtain the work done in joules.

Section 3 of 5Energy Transfers

Energy Transfers
Braking
  • Work done by the brakes transfers kinetic energy of the car into the thermal store of the brakes.
Meteorite
  • Friction with air transfers kinetic energy into the thermal stores of the meteorite and the surrounding air.
Lifting
  • Work done against gravity transfers energy into the gravitational potential store of the lifted object.
No work
  • A normal contact force acting at right angles to horizontal motion on level ground does no work.

Section 4 of 5Worked Example

Question

  • A crate is pushed 12 m across a level floor by a constant horizontal force of 85 N.
  • Calculate the work done by the pushing force.

  1. Identify the valuesF = 85 N and d = 12 m, both in the direction of the force.
  2. Write the equation W = F × d.
  3. SubstituteW = 85 × 12 = 1020 J.
  4. Write the answer to two significant figures: 1.0 × 10³ J.

Answer The work done by the pushing force is 1.0 × 10³ J (1020 J).

Section 5 of 5Exam Checklist & Tips

2.1.1 – Key Points for the Exam

  • Work done Energy transferred by a force that moves an object through a distance in the direction of the force.
  • Joule (J) The SI unit of work and energy, equal to 1 N m.
  • Energy transfer The movement of energy from one store to another by a process such as work done.
  • Thermal store The store of energy an object has because of its temperature.
  • Gravitational potential store The store of energy an object has because of its height in a gravitational field.

Common Mistakes

  • Stating that work is done whenever a force is applied, even if the object does not move.
  • Using the total distance travelled when the force acts over only part of the path.
  • Forgetting that work is done only in the direction of the force.
  • Confusing work done with power; power is the rate at which work is done.
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