Physics · Grades 8–11
Work, transfer and energy accounting
Track work by each force and distinguish energy from power.
Learn the idea
Work
For a constant force, work is force times displacement times the cosine of their angle.
Sign
A force in the direction of displacement does positive work; an opposing force does negative work.
Perpendicular force
A force perpendicular to displacement does zero work in this model, even when the force is nonzero.
Energy account
Transferred energy goes somewhere. Dissipated energy is not destroyed, though it may become less useful.
Worked examples
A 12 N constant force acts along a 3 m displacement. Find its work.
W = Fd = 12 × 3 = 36 J.
36 J
A 5 N resisting force opposes a 4 m displacement. Find work by that force.
The opposing direction gives W = −5 × 4 = −20 J.
-20 J
Check your understanding
A force is perpendicular to displacement. What is its work in this model?
- Always equal to force
- Zero
- Always positive
Which is an energy unit?
- Joule
- Watt
- Metre per second
Explain why a person can feel tired holding a bag even if the bag's displacement is zero.
Answers and marking notes
Answers and marking notes
- Zero
The angle cosine is zero.
- Joule
A joule measures energy; a watt measures a rate of energy transfer.
- 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.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.