Stores of motion and position

1. An energy store describes where energy is held, and the amount of energy in each store is measured in joules, $J$.
2. Describing a change to a system means saying which stores decrease and which stores increase, from the start of the process to the end.
3. The kinetic store holds energy because an object is moving, so it increases when the object speeds up and decreases when it slows down.
4. A heavier object holds more energy in its kinetic store than a lighter one moving at the same speed, so a loaded lorry at $20m/s$ holds far more than a cyclist at $20m/s$.
5. The gravitational potential energy store holds energy because of an object's height in a gravitational field, so it increases when the object is raised and decreases when it falls.
6. This store belongs to the object and the Earth together, because it depends on the gravitational attraction between them.
7. The elastic potential energy store holds energy when an object is stretched, squashed or bent and can return to its original shape, such as a stretched bungee cord or a compressed spring.
8. These three stores change whenever an object speeds up, slows down, rises, falls, stretches or springs back.

Stores of particles and fields

1. The thermal store of an object increases when its temperature rises, because its particles vibrate or move faster.
2. A large object at a low temperature can hold more energy in its thermal store than a small object at a high temperature, so a swimming pool at $25{}^{\circ }C$ holds far more than a mug of tea at $80{}^{\circ }C$.
3. The chemical store is found in fuels such as petrol and natural gas, in food and in batteries, and it decreases when a chemical reaction such as burning or respiration takes place.
4. The nuclear store is held in the nuclei of atoms and decreases when nuclei split apart in a nuclear power station or join together in the Sun.
5. The magnetic store changes when magnets or magnetic materials that attract or repel each other move closer together or further apart, as when two fridge magnets snap together.
6. The electrostatic store changes when electric charges that attract or repel each other move closer together or further apart, as when a charged balloon is pulled towards a wall.
7. Electricity, light and sound are not stores, because they are ways of transferring energy from one store to another.

Projecting an object upwards

1. When you throw a ball straight up, your muscles transfer energy from the chemical store in your body to the kinetic store of the ball.
2. As the ball rises it slows down, so its kinetic store decreases while its gravitational potential energy store increases.
3. At the top of its flight the ball is momentarily still, so its kinetic store is empty and its gravitational potential energy store is at its largest.
4. As the ball falls back down, its gravitational potential energy store decreases and its kinetic store increases, so it speeds up.
5. Air resistance acts on the ball throughout its flight and increases the thermal stores of the ball and the air, so the ball returns to your hand slightly slower than it left.
6. An object pushed up a slope behaves in the same way, with its kinetic store decreasing and its gravitational potential energy store increasing as it climbs.
7. Friction between the object and the slope also increases the thermal stores of both surfaces, so a trolley pushed up a ramp stops lower down than it would on a perfectly smooth ramp.

Hitting an obstacle

1. A moving object that hits an obstacle, such as a car hitting a crash barrier or a hammer hitting a nail, loses most or all of its kinetic store in a fraction of a second.
2. During the impact the object and the obstacle are squashed or bent, and the energy is transferred to their thermal stores and carried away to the surroundings as sound.
3. If the change of shape is elastic, as when a tennis ball hits a wall, some energy is held briefly in the elastic potential energy store and then returned to the kinetic store as the ball bounces away.
4. If the change of shape is plastic, as in the crumple zone of a car, the shape change is permanent and the energy ends up in thermal stores rather than being returned.
5. When a hammer drives a nail into wood, the kinetic store of the hammer decreases and the thermal stores of the nail, the hammer head and the wood increase, which is why the nail feels warm afterwards.
6. The sound produced by any impact also ends up in the thermal store of the surrounding air once the sound waves have died away.

Accelerating with a constant force

1. When a constant resultant force acts on an object in its direction of motion, the object speeds up, so its kinetic store increases steadily.
2. The energy always comes from the store that supplies the force, such as the chemical store of a car's petrol, a cyclist's muscles or the battery in an electric scooter.
3. An electric car pulling away from traffic lights transfers energy from the chemical store of its battery to its kinetic store.
4. Not all of that energy reaches the kinetic store, because the motor, the tyres and the air all warm up, so their thermal stores increase too.
5. A rocket at launch shows several changes at once: the chemical store of its fuel decreases while its kinetic store and gravitational potential energy store both increase.

Slowing a vehicle down

1. When a driver brakes, the brake pads press against discs fixed to the wheels, and friction between them acts against the motion.
2. The kinetic store of the car decreases as it slows down.
3. The thermal stores of the brake pads and discs increase, and the discs can glow red after repeated hard stops from high speed.
4. Energy then spreads from the hot brakes to the thermal store of the surrounding air, and squealing brakes or tyres also carry some energy away as sound.
5. Air resistance and friction in the moving parts act on a vehicle all the time, so a car that coasts with its engine off also slows down and its kinetic store decreases.
6. A cyclist who stops pedalling on a flat road shows the same change more slowly, with the kinetic store going to thermal stores in the air, the tyres and the bearings.

Boiling water in a kettle

1. An electric kettle is plugged into the mains, so energy is transferred electrically from the mains supply to the kettle's heating element.
2. The thermal store of the element increases, and because the element is hotter than the water, energy is transferred by heating to the water around it.
3. The thermal store of the water increases and its temperature rises steadily from room temperature towards $100{}^{\circ }C$.
4. Once the water reaches $100{}^{\circ }C$, its temperature stops rising and the energy supplied turns some of the liquid water into steam.
5. The plastic body of the kettle, its lid and the air in the kitchen also warm up, so their thermal stores increase as well.
6. Steam escaping from the spout carries energy into the room, and the rumbling sound of the boiling water carries a little more away.
7. In every one of these situations, the decreases and increases balance, so the total energy is unchanged once all the stores are included.