Work

Meaning of Work Done (Physics Definition)

In physics, work is done when:

• a force acts on an object, and

• the object moves in the direction of the force.

If there is no movement in the direction of the force, no work is done (even if the force is large).

Formula for Work Done

Where:

• W = work done (joules, J)

• F = force applied (newtons, N)

• s = distance moved in the direction of the force (metres, m)

Unit check:

Direction Matters (Exam-Critical)

Only the distance moved in the direction of the force counts.

• Force applied ⟂ motion → no work done by that force.

• Force opposite motion → negative work (treated qualitatively at this level).

[Insert diagram showing a force acting along the direction of motion]

Everyday Interpretation

• Pushing a box across the floor → work is done.

• Holding a heavy bag still → no work done (no movement).

• Lifting an object vertically → work is done against gravity.

[Insert diagram showing lifting an object vertically]

Link Between Work and Energy

Work done on an object results in:

• an increase in energy of the object, or

• a transfer of energy from one object/system to another.

Example:

• Work done lifting an object → increase in gravitational potential energy.

• Work done accelerating an object → increase in kinetic energy.

Worked Examples (Teaching Core)

Example 1: Simple Work Calculation

A force of 10 N moves a box 4 m along the floor.

Calculate the work done.

Solution

Example 2: Lifting an Object

A student lifts a 2 kg object vertically through 3 m.

(Take g = 10 m s.)

Step 1: Find the force used (weight)

Step 2: Calculate work done

Example 3: No Work Done Case

A person pushes a wall with a force of 50 N, but the wall does not move.

Answer:

No work is done because there is no displacement.

Key Examination Tips (High-Value)

• Always check movement occurs.

• Use metres (not cm).

• Use newtons (not kg).

• Write the formula first to secure method marks.

• State the unit (J) in the final answer.

Common Examination Errors (Examiner Insight)

Students often:

• say work is done when holding an object still,

• forget that distance must be in the direction of force,

• confuse force with mass,

• omit units.

Clear reasoning + correct units = easy marks.

Exam-Style Questions (Original)

Question 1

State the formula used to calculate work done.

Question 2

A force of 15 N moves an object 6 m in the direction of the force.

Calculate the work done.

Question 3

Explain why no work is done when a person holds a heavy suitcase without moving.

Question 4

A box is pulled along the floor by a force of 20 N for 5 m.

Calculate the work done and state the unit.

Worked Solutions (Beyond Excellent)

Solution 1

Solution 2

Solution 3

No work is done because the suitcase does not move, so the distance moved is zero.

Solution 4

End-of-Objective

A learner who has mastered this objective can:

• define work done correctly,

• relate work to force and distance,

• apply $W = F \times s$ accurately,

• link work done to changes in energy.

Core Idea: Work as Energy Transfer

Work and energy are closely linked:

Work done is the transfer of energy from one object or system to another.

• When work is done on an object, the object gains energy.

• When an object does work on its surroundings, it loses energy.

This relationship is universal across mechanics.

Energy Change Equals Work Done (Conceptual Form)

For many mechanical situations (ignoring losses):

This means:

• Positive work → increase in energy

• Negative work → decrease in energy

Work Done and Kinetic Energy

When a force causes an object to speed up, work is done on the object.

• The work done increases the object’s kinetic energy.

• Greater work → greater increase in speed.

[Insert diagram showing a force accelerating a block, with KE increasing]

Example (conceptual):

• Pushing a trolley makes it move faster.

• Work done by the push increases the trolley’s kinetic energy.

Work Done Against Gravity and Potential Energy

When an object is lifted vertically:

• work is done against gravity,

• the object gains gravitational potential energy.

Here:

[Insert diagram showing lifting a load vertically and gaining GPE]

Work Done Against Friction and Energy Dissipation

When work is done against friction:

• energy is transferred mainly into thermal energy,

• the object or surroundings become warmer.

Although energy seems “lost”, it is actually converted, not destroyed.

[Insert diagram showing sliding object with friction producing heat]

No Work Done, No Energy Transfer

If:

• a force acts, but

• there is no movement in the direction of the force,

then:

• no work is done, and

• no energy is transferred by that force.

Example:

• Holding a heavy suitcase still causes no work to be done on the suitcase.

Summary of Relationships (Exam-Ready)

Common Examination Errors (Examiner Insight)

Students often:

• define work and energy separately without linking them,

• say energy is “used up” instead of transferred,

• forget to mention direction of motion,

• confuse work done with force applied.

Always link work → energy transfer/change.

Exam-Style Questions (Original)

Question 1

State the relationship between work and energy.

Question 2

Explain how lifting an object shows the relationship between work and energy.

Question 3

Describe what happens to energy when work is done against friction.

Question 4

A force acts on an object but no work is done.

Explain how this is possible.

Worked Solutions (Beyond Excellent)

Solution 1

Work done is the transfer of energy from one object or system to another.

Solution 2

When an object is lifted, work is done against gravity and this work results in an increase in gravitational potential energy.

Solution 3

Work done against friction transfers energy into thermal energy, warming the object and its surroundings.

Solution 4

If the object does not move in the direction of the force, the distance moved is zero, so no work is done and no energy is transferred.

End-of-Objective

A learner who has mastered this objective can:

• explain work as energy transfer,

• relate work done to changes in KE and GPE,

• describe energy dissipation due to friction,

• answer descriptive and explanatory exam questions confidently.