Energy, Work, and Power Essentials
Key formulas and exam skills for work, energy stores, power, and efficiency.
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Questions Covered in This Set
10 cards to master
Formula for work done
W = F × d, where d is the distance moved in the direction of the force. Work is measured in joules (J).
Why is little work done carrying a box horizontally?
The upward force you apply is perpendicular to the motion, so there is no distance moved in the direction of that force — work ≈ 0 J.
Gravitational potential energy formula
Ep = mgh — mass (kg) × gravitational field strength (9.8 N/kg, sometimes 10) × VERTICAL height (m).
Kinetic energy formula and the effect of doubling speed
Ek = ½mv². Because v is squared, doubling the speed quadruples the kinetic energy.
Principle of conservation of energy
Energy cannot be created or destroyed, only transferred between stores. 'Lost' energy is usually dissipated as heat/sound to the surroundings.
Speed of a dropped object from height h (no air resistance)
mgh = ½mv², so mass cancels: v = √(2gh). A heavier ball does NOT land faster.
A 0.50 kg ball dropped from 20 m — speed on landing?
Ep = 0.50 × 9.8 × 20 = 98 J; v² = 2 × 98 / 0.50 = 392; v ≈ 19.8 ≈ 20 m/s.
Power: definition, formulas, and unit
Power is the rate of energy transfer. P = E/t = W/t, and P = Fv. Measured in watts (W), where 1 W = 1 J/s.
Efficiency equation
Efficiency = (useful output energy or power ÷ total input energy or power) × 100%.
Exam unit-conversion checklist
grams → kg (÷1000), km → m (×1000), km/h → m/s (÷3.6), minutes → seconds (×60). Convert BEFORE substituting.
