Forces, motion, energy, work and power
This domain links forces to motion and energy to work and power. The exam asks for calculations with correct units, graph interpretation, and descriptions of energy using stores and transfers rather than 'using up'.
Forces and motion
A force is a push or pull, measured in newtons (N). Weight = mass × gravitational field strength (W = mg, g ≈ 9.8 N/kg on Earth). Mass in kg is constant; weight changes with gravity. Friction opposes motion and generates heat. Forces are balanced when the resultant is zero, giving rest or constant velocity; unbalanced forces cause acceleration (F = ma). Speed = distance ÷ time in m/s. The slope of a distance–time graph gives speed; the slope of a velocity–time graph gives acceleration and the area under it gives distance.
Energy, work and power
Energy is stored or transferred, never created or destroyed. Stores: kinetic (½mv²), gravitational potential (mgh), elastic, chemical, thermal, nuclear, magnetic, electrostatic. Transfers: heating, light, sound, electrical, mechanical. Work = force × distance (J). Power = work ÷ time (W). Efficiency = useful output ÷ total input × 100%.
Worked example
A 50 kg student climbs 4 m of stairs in 5 s.
- Gain in gravitational potential energy = mgh = 50 × 9.8 × 4 = 1960 J.
- Power = energy ÷ time = 1960 ÷ 5 = 392 W.
If she takes 10 s, the energy gained is the same but the power halves, because power is the rate of energy transfer.
Traps
- Giving weight in kg or mass in N.
- Failing to convert cm to m or g to kg.
- Reading distance directly off a velocity–time graph instead of taking the area.
- Describing energy as 'used up' rather than transferred to a store.