A system is an object or a group of objects. When a system changes, energy is moved between the energy stores of the system. The stores you need are kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear.
You should be able to describe which stores change in a given situation:
- A ball thrown upwards: the kinetic store decreases and the gravitational potential store increases as it rises.
- A stretched spring released: energy moves from the elastic potential store to the kinetic store as the spring pulls back.
- A car braking to a stop: the kinetic store of the car decreases and the thermal store of the brakes increases.
- A battery lighting a torch: the chemical store of the battery decreases and the thermal store of the surroundings increases.
- Fuel burning in a car engine: the chemical store of the fuel decreases, the kinetic store of the car increases, and some energy is transferred to the thermal store of the surroundings.