Work, Energy and Power
Calculate work done, kinetic and potential energy, and power.
Work is done when a force moves an object through a distance.
- Calculate work done.
- Distinguish kinetic and potential energy.
- Calculate power and efficiency.
Work
Work = force × distance moved in the direction of the force. Work is measured in joules (J). Lifting a 10 N load by 2 m does 20 J of work.
Energy
Energy is the ability to do work. Kinetic energy is energy of motion: KE = ½mv². Gravitational potential energy is stored energy: PE = mgh.
Power and Efficiency
Power = work ÷ time, measured in watts (W). Efficiency = useful energy output ÷ total energy input × 100%.
A 20 N force moves a box 5 m: work = 20 × 5 = 100 J.
A 2 kg object moves at 3 m/s: KE = ½ × 2 × 3² = 9 J.
A machine does 600 J in 20 s: power = 600 ÷ 20 = 30 W.
Practice
- Calculate the work done lifting a 50 N bag through 1.5 m.
- Find the kinetic energy of a 4 kg object moving at 5 m/s.
- A motor does 1 200 J of work in 4 s. Find its power.
Recap
Work transfers energy. Power is the rate of transfer, and efficiency compares useful output with total input.
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Forces and Motion
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