- Generally speaking energy can either be useful or wasted. For example energy can be used to lift a weight but energy can be lost as heat because of friction.
- The useful bit of energy is called work and if we lift a weight we are said to have 'done work'. Work is linked to force by the following equation:
- In this equation force is measured in newtons (N), distance is measured in meters (m) and the work done is measured in joules (J), which is the unit normally used for any energy.
- To lift an object you need to over come the force of gravity with an equal force acting is in the opposite direction. For this reason the 'force for movement' will be equal but opposite to the force opposing the movement.
- It is important to note the difference between mass and weight. An object's mass is due to the amount of matter it contains and has the units of kilograms (kg). Weight is the force that an object exerts downwards towards the ground and is measured in newtons (N).
- The weight of an object can be calculated by multiplying its mass by the acceleration due to gravity which is 9.8 meters per second per second (often rounded up to 10 m/s2).
- How much energy (or how much work) is needed to lift a 70kg man off the floor to a height of 3m (using the acceleration due to gravity = 10 m/s2).
- Power is the rate of using energy or doing work. Lifting the man in the previous example to 3m will always require the same amount of work but the quicker you do it, the more power is needed.
P = W/t
- The unit of power is the watt (W, not to be confused with the symbol for work).
- One watt is the power used supplying one joule per second. The symbol W is also used for energy used or work done, so be careful not to get confused with W used as the units of power.
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