When a system changes, energy is moved from one store to another. You need to be able to describe these changes for some common situations.
- Object projected upwards, or up a slope: the kinetic store decreases and the gravitational potential store increases. A little energy is also transferred to the thermal store by air resistance or friction.
- Object accelerated by a constant force: the force does work on the object, and energy is transferred to its kinetic store, so it speeds up.
- Vehicle slowing down: the kinetic store falls. Friction in the brakes does work, so the thermal store of the brakes increases.
- Moving object hitting an obstacle: the kinetic store falls. Energy is transferred to the thermal store of the object and its surroundings, and away by sound waves.
- Electric kettle: an electric current transfers energy from the mains to the thermal store of the water, which rises in temperature until it boils.
In each case, the energy lost from some stores equals the energy gained by others.