Energy stores
Energy can be held in several stores. Common ones are: kinetic (moving objects), gravitational potential (raised objects), elastic (stretched or squashed springs), chemical (fuels, food, batteries), thermal/internal (hot objects), nuclear, electrostatic and magnetic.
Transfers and conservation
Energy moves between stores by transfer pathways: mechanically (by a force), electrically (by a current), by heating, and by radiation (light, sound, waves). The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another. The total energy always stays the same.
Key idea
Kinetic energy: Ek = ½ m v². Change in gravitational PE: ΔEp = m g Δh, with g ≈ 9.8 N/kg (use 10 N/kg if told).
Worked example
A 0.20 kg ball is dropped from a height of 5.0 m. Using g = 10 N/kg, find its speed just before it lands (ignore air resistance).
GPE lost = m g Δh = 0.20 × 10 × 5.0 = 10 J.
This transfers to kinetic energy: ½ m v² = 10 J.
½ × 0.20 × v² = 10, so v² = 100 and v = 10 m/s.
Remember
- Stores hold energy; pathways transfer it.
- Total energy is always conserved.
- Some energy is usually dissipated to the surroundings as thermal energy.