IGCSE Physics / Energy transfers
Conservation of energy revision notes
Edexcel International GCSE Physics (4PH1), chapter 14: Energy transfers, specification point 4.3. Free lesson.
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Energy cannot be created or destroyed, only transferred between stores. Conservation of energy lets you track useful and wasted energy, including why a ball bounces lower each time.
What conservation of energy means
Energy can move from one store to another, but the total amount stays the same. This means the total energy going in must equal the total energy coming out. Energy cannot be created or destroyed. Total energy in = total energy out.
How energy transfers in a pendulum
As a pendulum swings, energy transfers between its gravitational and kinetic stores. At the top of the swing, the energy is all gravitational. At the bottom, it is all kinetic. At other points, it is shared between the two stores, but their total stays constant. In a model with no air resistance, the total energy remains constant as the pendulum swings. A real pendulum gradually slows because energy is transferred by heating to the thermal store of the air. The total energy is still conserved.
Why a bouncing ball gets lower
A ball held high has energy in its gravitational store. As it falls, energy transfers to its kinetic store. When it hits the floor and squashes, energy transfers to its elastic store, then back to kinetic as it springs up. Some energy transfers by heating and sound to the ball, floor and surroundings. So less remains in the ball's gravitational store for the next bounce.
How to calculate wasted energy
The energy supplied is split between useful energy and wasted energy. Rearrange the conservation equation by subtracting the useful energy from the energy in. Energy in = useful energy out + wasted energy out Wasted energy is often transferred to the thermal store of the surroundings.
Where energy goes when a ball stops
Energy is not used up or lost when a ball stops bouncing. It has transferred to the thermal store of the ball, floor and air, where it is spread out and less useful. When energy moves to other stores, the total still adds up to the amount the ball started with. For a dropped ball, the energy transferred to other stores is the starting gravitational energy minus the gravitational energy at the top of the bounce. Name a store the energy transferred to, such as the thermal store of the ball, floor or surroundings.