GCSE Physics: Energy stores and transfers

Conservation of energy, kinetic and gravitational potential stores, efficiency.

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Can energy be created or destroyed?
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No. Energy can only be transferred from one store to another or dissipated to the surroundings.
What happens to the total energy of a closed system?
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It always stays the same.
What is the equation for kinetic energy?
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kinetic energy = ½ × mass × speed²
If an object's speed is doubled, how does its kinetic energy change?
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It quadruples (becomes four times greater), because kinetic energy depends on the square of the speed.
What is the equation for gravitational potential energy?
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gravitational potential energy = mass × gravitational field strength × height
What happens to an object's gravitational potential energy when it falls?
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It is transferred to the kinetic store.
Why is some energy always dissipated in real energy transfers?
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Usually because of friction, which transfers energy to thermal energy in the surroundings where it spreads out and becomes less useful.
Define efficiency of a device.
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The fraction of the input energy that is transferred to a useful output store.

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