Energy is also stored in stretched elastic or springs. The stretched elastic of a catapult has stored energy, which can be used to move a stone. This is called strain or elastic potential energy.
When we use energy to do something that energy is not used up or destroyed but it changes or transfer’s from one form to another.
The unit used to measure all types of energy is the joule (J).
One joule is very small, about the energy needed to life a book 10cm. A packet of crisps or a small chocolate bar each contain about 600,000 joules of chemical energy.
1 kilojoule (kJ) = 1000 joules.
Although energy can be changed from one form to another, the amount of energy stays the same, as energy cannot be created or destroyed. Meaning whatever the forms of energy involved, total energy output = total energy input.
Total energy input = useful energy output + wasted energy output
The efficiency of a device is the fraction of the energy supplied to the device, which is usefully transferred.
% Efficiency = useful energy transferred by device divided by total energy supplied to device X 100%
The useful energy got out is usually less than the total energy put in, due to friction in most machines, and so efficiency is less than 100%. It cannot be greater than 100%!
Is someone is doing work there is movement against an opposing force.
Energy is transferred to the object that person is moving.
The amount of energy transferred depends on the size of the force and the distance the object moves in the direction of the force.
If the person pushes against a larger force or moves it a greater distance she will do more work.
Work done (J) = force (N) X distance moved (M)
If a force acts on an object but does not move it, there is no energy transferred to the object and no work is done.
Power is a measure of fast energy is transferred. The greater the power, the more energy is transferred in a given time.
Power is measure in watts (W)
Power (W) = work done (J) divided by time taken (S)
1 watt is a rate of working of 1 joule per second (1J/s)
When you move an object upwards, you do work against the force of gravity. The work done (energy transferred) is stored in the object as gravitational potential energy. Because the object is higher, it can release energy by falling down again.
The higher you life the object, or the greater the force needed to move the object, the more work you do and do the more gravitational potential energy is transferred to the object.
The force of gravity which you must overcome to life the object is usually called the objects weight.
The weight of an object depends on its mass (the amount of matter in the object) and the gravitational field strength (g)
Weight (N) = mass (kg) X gravitational field strength (N/kg)
To lift the object a vertical height, the work which must be done on the object, (energy transferred) is weight X vertical height.
Gravitational potential energy gained (J) = weight (N) X vertical height raised (m)
= Mass (kg) X gravitational field strength (N/kg) X height (m)
When work is done to change the shape of an object, the energy transferred is stored as elastic potential energy.
The greater the force needed to squash a spring or to stretch elastic or a spring, and the greater the movement (compression or extension), the more work is done, and so more energy is transferred.
Kinetic energy is the energy an object has due to its movement. The greater the mass of the object and the greater its speed, the more kinetic energy it has.
Kinetic energy, mass and speed are related by:
Kinetic energy (J) = ½ mass (kg) X (speed) (m/s)
