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What does it mean, in physics, to do work, and how is work related to energy?

Define work done, apply work equals force times distance, and link work to the transfer of energy

A focused answer to the O-Level Physics outcome on work. Work as force times distance moved in the direction of the force, the joule, when no work is done, and the link between work done and energy transferred.

Generated by Claude Opus 4.87 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to define work done, to use the relationship W=F×dW = F \times d, to know that the unit of work is the joule, and to link work done to the transfer of energy. The big idea is that work is done only when a force moves an object in the direction of the force, and the work done is the energy transferred.

The answer

What work means in physics

Work is done when a force moves an object in the direction of the force. The work done equals the force times the distance moved in the direction of the force:

W=F×dW = F \times d

Force is in newtons and distance in metres, so the unit of work is the joule (J\text{J}), where 1 J=1 N m1\ \text{J} = 1\ \text{N m}. One joule is the work done when a force of one newton moves an object one metre.

Work and energy are linked

Doing work transfers energy. When you do work on an object, you transfer energy to it; the work done equals the energy transferred:

work done=energy transferred\text{work done} = \text{energy transferred}

Lifting a box does work against gravity and stores gravitational potential energy. Pushing a trolley does work and gives it kinetic energy.

When no work is done

No work is done if either the force or the distance moved in the direction of the force is zero.

  • Holding a heavy bag still: the bag does not move, so no work is done on it (even though it feels tiring).
  • Carrying a bag horizontally at steady speed: the lifting force is vertical but the motion is horizontal, so that force does no work.

Same unit as energy

Because work done equals energy transferred, work and energy share the same unit, the joule. This is the bridge between this dot point and the energy forms.

Examples in context

Example 1. Climbing stairs. Walking up a flight of stairs you do work against gravity, lifting your weight through the height of the stairs. That work is stored as gravitational potential energy, which is why climbing is tiring and why you can feel the energy released if you then run back down.

Example 2. A satellite in orbit. A satellite moving in a circle is pulled by gravity toward the centre, but it moves perpendicular to that pull. Because the force and the motion are at right angles, gravity does no work on it, so its speed (and kinetic energy) stays constant as it orbits.

Try this

Q1. A force of 40 N40\ \text{N} moves a box 3.0 m3.0\ \text{m} in the direction of the force. Calculate the work done. [2 marks]

  • Cue. W=F×d=40×3.0=120 JW = F \times d = 40 \times 3.0 = 120\ \text{J}.

Q2. State the unit of work and explain how it relates to energy. [2 marks]

  • Cue. The joule (J\text{J}); work done equals the energy transferred, so work and energy share the same unit.

Q3. Explain why no work is done when a person holds a heavy box still. [2 marks]

  • Cue. Work is force times distance moved in the force's direction; with no movement the distance is zero, so no work is done.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA worker pushes a crate 6.0 m6.0\ \text{m} across a floor with a steady horizontal force of 80 N80\ \text{N}. (a) Calculate the work done. (b) State the energy transferred to the crate and surroundings.
Show worked answer →

(a) Work done: W=F×d=80×6.0=480 JW = F \times d = 80 \times 6.0 = 480\ \text{J}.

(b) The work done equals the energy transferred, 480 J480\ \text{J}. Some becomes kinetic energy of the crate, and the rest is transferred to heat by friction between the crate and the floor.

Markers reward work as force times distance, the answer in joules, and the point that the work done equals the energy transferred.

Original4 marksA weightlifter holds a 600 N600\ \text{N} barbell still above his head for 5 s5\ \text{s}. (a) State the work done on the barbell during this time, and explain your answer. (b) He then lowers it 0.50 m0.50\ \text{m} to his chest under control. State the work done by gravity on the barbell during the lowering.
Show worked answer →

(a) The work done is zero. Work is force times distance moved in the direction of the force; while the barbell is held still it does not move, so no work is done on it, even though the lifter feels tired.

(b) Gravity acts downward and the barbell moves 0.50 m0.50\ \text{m} downward, so gravity does positive work: W=F×d=600×0.50=300 JW = F \times d = 600 \times 0.50 = 300\ \text{J}.

Markers reward zero work when there is no movement, and work as force times distance (300 J300\ \text{J}) for the lowering, with the recognition that gravity and the motion are in the same direction.

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