Work Notes, Quizzes & Revision
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Notes: Subtopic — Work
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What is work? (simple definition)
In physics, work is done when a force causes an object to move in the direction of the force. In everyday Kenyan life: when a market trader lifts a crate of tomatoes, when a Jua Kali welder pushes a piece into position, or when you carry a jerrycan of water across the compound, work is being done.
Mathematical formula
where θ is the angle between the force direction and the displacement direction.
Special cases:
- If force and displacement are in the same direction: W = F × d (θ = 0° so cosθ = 1).
- If force is perpendicular to displacement (θ = 90°): W = 0 (no work done by that force).
- Units: Force in newtons (N), displacement in metres (m), work in joules (J). 1 J = 1 N·m.
Examples with Kenyan context
A woman lifts a 10 kg jerrycan of water from the ground to the top of a 1.2 m well. Assume g = 9.8 m/s².
Solution:
- Force needed (weight) F = m × g = 10 kg × 9.8 m/s² = 98 N.
- Displacement d = 1.2 m, force and displacement are vertical and same direction → θ = 0°.
- Work W = F × d = 98 N × 1.2 m = 117.6 J ≈ 118 J.
A trader pushes a crate with a horizontal force of 50 N for 4 m across the stall floor. Force is along the displacement (θ = 0°).
Solution:
W = F × d = 50 N × 4 m = 200 J.
If friction resists movement, the net work by the pushing force is still 200 J, but some of that energy becomes thermal due to friction.If you hold a bag and walk horizontally at constant height, the upward force you exert (to support the bag) is perpendicular to the horizontal displacement → Work by that upward force = 0.
Work and energy — short link
Work changes the energy of an object. The work-energy theorem states:
Signs of work (positive, negative, zero)
- Positive work: Force has a component in the direction of motion (speed increases or energy added).
- Negative work: Force acts opposite to motion (brakes on a matatu do negative work, reducing kinetic energy).
- Zero work: Force perpendicular to motion or no displacement (holding a bucket still → no work by your upward force).
Worked practice questions (with answers)
Answer: F = 5×9.8 = 49 N → W = 49×0.8 = 39.2 J ≈ 39 J.
Answer: W = 30×10 = 300 J.
Key terms (quick glossary)
- Force: A push or pull (measured in N).
- Displacement: How far and in which direction an object moves (m).
- Work: Product of force and displacement in the direction of force (J).
- Joule (J): Unit of work or energy (1 N·m).
- Work-energy theorem: Net work = change in kinetic energy.
Simple visual: pushing box
Summary
Work measures energy transfer when a force moves an object. Use W = Fd cosθ, keep units N, m and J, and connect work to changes in energy. Think of everyday Kenyan examples (lifting water, pushing crates, riding boda-boda) to understand and practise calculations.