This is the example we used.
We had to find:
a) The total energy
b) The speed of the man 8m above the ground.
Answer:
a) The man at the top of the building is not moving so all of the energy he possesses is potential. This means that the total energy this man will ever have is equal to the potential energy at the top of the building as shown below.
b) At 8m above the ground the man now has some kinetic energy as he is falling and some potential energy as he is still above the ground. The total energy the man possesses hasn't changed, only the way the energy is distributed. Therefore the potential energy and the kinetic energy must add up to the total. By working out the potential energy the man has a 8m we can then work out the kinetic energy as shown below. When the kinetic energy is known, the velocity can be found, as shown below.
Other Notes:
Other things people found confusing was the input energy produced by a fuel like petrol. Fuel has an energy either per litre or per kg. Therefore the amount of fuel used must be multiplied by the energy stored in the fuel.
So if petrol has 3.5MJ of energy per kg, and uses 2kg of fuel then the total energy supplied by the fuel is 3.5MJ x 2 = 7MJ.
People also found it difficult to work out which was the input energy and which was the output energy. The only real rule in trying to work this out is to read the question carefully. It will explain in the question how the system works. It may also give you a diagram. As a rule of thumb a system diagram will read input > process > output. Therefore a diagram should be drawn in this order.
Other than the above: read the question, highlight and label all the relevant information so that it is clear what you know, and what you are aiming to find out. Use the data booklet to help you work out what equation you need to use based on what information you have.
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