A 12 N constant force acts along a 3 m displacement. Find its work.
Answer and explanation
36 J
W = Fd = 12 × 3 = 36 J.
Subjects
Energy, work & power
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Sample exercises
A 5 N resisting force opposes a 4 m displacement. Find work by that force.
Answer and explanation
-20 J
The opposing direction gives W = −5 × 4 = −20 J.
A force is perpendicular to displacement. What is its work in this model?
- Always equal to force
- Zero
- Always positive
Answer and explanation
Zero
The angle cosine is zero.
Which is an energy unit?
- Joule
- Watt
- Metre per second
Answer and explanation
Joule
A joule measures energy; a watt measures a rate of energy transfer.
Explain why a person can feel tired holding a bag even if the bag's displacement is zero.
Answer and explanation
Mechanical work on the stationary bag is zero, but the person's body can still use internal energy.
Credit distinction between mechanical work on the bag and biological energy use.
A machine receives 200 J and transfers 140 J usefully. Find the remaining energy transfer.
Answer and explanation
60 J
200 − 140 = 60 J must be included elsewhere in the energy account.
Study notes
Work, transfer and energy accounting
Track work by each force and distinguish energy from power.
Grades 8–11Work, transfer and energy accounting · Revision sheet
Track work by each force and distinguish energy from power.
Grades 8–11Kinetic and gravitational potential energy
Check squared speed, vertical height and energy reference levels.
Grades 8–11Kinetic and gravitational potential energy · Revision sheet
Check squared speed, vertical height and energy reference levels.
Grades 8–11Power and efficiency without unit confusion
Compare rates and fractions rather than mixing energy and power.
Grades 8–11Power and efficiency without unit confusion · Revision sheet
Compare rates and fractions rather than mixing energy and power.
Grades 8–11Work by an angled force
θ is the angle between the force and displacement, not an arbitrary drawing angle.
Grades 10–12Change in kinetic energy
Square each speed before subtracting; a loss gives a negative change.
Grades 9–12Gravitational energy gain
Use the vertical height change and the specified constant field strength.
Grades 7–10Useful output energy
Convert the efficiency percentage to a fraction; wasted energy remains part of the energy account.
Grades 8–11Energy from constant power
Power is an energy-transfer rate; watts times seconds gives joules.
Grades 7–10Elastic energy in a spring
Convert extension to metres, then square it; the spring is ideal and linear.
Grades 10–12Energy, work & power · Formula reference 1
Formula reference
Grades 7–12Energy, work & power · Formula reference 2
Formula reference
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