E Energy Energy Calculating energy Example: Calculating landing speed

Energy We say someone has a lot of energy when they run and do lots of things. Scientists define energy as ‘the ability to do work’. Work is done when a force moves something. You should already be familiar with the following eight forms of energy: kinetic (movement) energy sound energy radiant (light) energy heat energy electrical energy gravitational potential energy elastic potential energy chemical potential energy

Energy can be changed from one form to another. Here elastic potential energy in the spring is converted to kinetic energy and then to gravitational potential energy.

This heater changes electrical energy into heat energy and radiant energy. The spirit burner changes chemical potential energy into heat energy and radiant energy.

When this tuning fork is hit, kinetic energy from the hammer is converted to sound energy.

This rollercoaster converts gravitational potential energy into kinetic energy and back into gravitational potential energy again. A small amount of energy is converted into heat due to friction with the rails and the air. Energy can neither be created or destroyed, it just changes form. Unless the car has been given an extra push, it cannot rise higher than its starting point.

Energy Energy is the ability to do work. Work is done when a force moves something. Different kinds of energy are: kinetic (movement) energy, sound energy, heat energy, radiant (light) energy, electrical energy, gravitational potential energy, elastic potential energy and chemical potential energy. Energy can be transformed or changed from one form into another. For example, an electric heater converts electrical energy into heat energy and radiant energy. Most forms of energy end up being converted into heat energy in the environment. Energy cannot be created or destroyed, only changed from one form into another. 1E 1 Energy transformations 1E 2 Energy

Calculating energy Gravitational potential energy If someone says you have the potential to get good marks, they mean you have the ability to do well, but haven’t actually done it yet. Potential means ‘stored up’, ‘yet to be used’. Objects have gravitational potential energy when they are high up. Energy was put in to lift them up, and this energy will be transformed again when the object falls. This climber has a lot of gravitational potential energy.

What factors determine the climber’s gravitational potential energy (Ep) in joules? His height (h) in metres – the higher he climbs, the more energy he had to put in, and the greater his Ep. His mass (m) in kilograms – the heavier he is, the more energy is needed to overcome gravity for the climb. The acceleration due to gravity (g) in m s–2 – it’d be much easier to do this climb on the Moon. Thus: ΔEp = m g Δh

Kinetic energy Kinetic energy (Ek) is the energy of moving objects. The greater an object’s speed, the greater its kinetic energy. If two objects are travelling at the same speed, the one with the greater mass has more kinetic energy. Where Ek is the kinetic energy in J, m is mass in kg, v is velocity in m s-1. Notice that when mass is doubled, Ek doubles, but when speed is doubled, Ek goes up by a factor of four (22).

Potential energy is stored energy. Calculating energy Potential energy is stored energy. Objects have gravitational potential energy when they have been lifted up. The amount of energy depends on their height, their mass, and the strength of the gravitational field. Objects have kinetic energy when they are moving. The amount of kinetic energy depends on their mass and speed. When mass doubles, Ek doubles, but when speed doubles, Ek goes up by a factor of four. m (mass) in kg h (height) in m Ep in J m (mass) in kg v (speed) in m s–1 Ep in J 1E 3 Kinetic energy trials

Example: Calculating landing speed A 3.5 kg cat climbs 1.8 m onto a fence then jumps off. What is its speed when it hits the ground? (Ignore air resistance, assume g = 10 m s−2.) Calculate the Ep at the top of the fence. This value is equal to its kinetic energy just before it lands. Ek = 63 J Calculate the speed from this Ek. 1E 4 Crash! 1E 5 Experimental design: stopping distances

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