Physics, Energy and Forces
Motion and forces
Distinguish scalars (speed, distance) from vectors (velocity, displacement). On a distance–time graph the gradient is speed; on a velocity–time graph the gradient is acceleration and the area beneath the line is distance travelled. Mass is the amount of matter (kg); weight is the force of gravity (N), calculated as W = m × g. Forces change shape, speed or direction; balanced forces give constant velocity, not necessarily rest.
Energy and efficiency
Energy is transferred between stores (kinetic, gravitational, elastic, thermal, chemical) and is conserved overall, but it becomes spread out and less useful - it is not "used up". Efficiency = useful output ÷ total input × 100%, always below 100% in real devices. Power is the rate of energy transfer, P = E ÷ t.
Waves and light
Waves transfer energy without transferring matter. Use v = f × λ. When a wave crosses into a new medium its speed and wavelength change but its frequency stays the same. Light bends towards the normal entering a denser medium and away from it leaving.
Electricity and magnetism
In series, current is the same everywhere and voltages add; in parallel, voltage is the same across each branch and currents add. Ohm's law: V = I × R. Resistance rises with length and temperature and falls with thickness.
Traps
Swapping mass and weight; reading area under a distance–time graph; saying energy is destroyed; thinking a renewable source has no environmental cost; assuming frequency changes on refraction.
Worked example
A lamp takes 500 J of electrical energy and transfers 300 J as light. Efficiency = 300 ÷ 500 × 100% = 60%; the remaining 200 J is dissipated as heat.