Power

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Power

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Objectives

• Know that power can be calculated as the rate that work is done.

• Be able to select and use the equation power = work done ÷ time

• Be able to use solve problems which combine ideas of mass, weight, work done and power.

Task 1

Remind the person next to you:

• What is meant by power?

• What is meant by a power of 5 Watts?

• What are the units of Work done?

Task 2

We know that power is the rate that energy is used - or to put it another way, the rate that work is done. We need to be able to calculate work done by a device, and then use this information to be able to calculate the power.

You met the formula for power during the "Core Science" course:

P= E ÷ t

P = Power, measured in Watts (W)

E = Energy transferred (or work done), measured in Joules (J)

t = time taken, measured in seconds (s)

Make a note of this formula in your exercise book, and show it as an equation triangle. You will also need the formula for work done from last lesson, and the formula which connects mass and weight.

Now, lets try calculating the power of these athletes. In each case you will need to calculate the work done first, and then use that information to calculate the power.

1. A weight-lifter takes 0.7 seconds to lift 60kg (600N) through 0.5m.

2. A fitness fanatic lifts 30kg (300N) through 0.6m. He repeats the exercise 12 times, which takes him 30 seconds.

3. Another fitness fanatic lifts 40kg through 0.8m. He repeats the exercise 20 times, which takes him 1.5 minutes.

4. A runner travels 800m against a frictional force of 60N, and takes 2 minutes.

5. A student has a mass of 50Kg and jumps 0.7m into the air. The student repeats this 10 times, which takes them 15 seconds.

Task 3

These problems combine all of the ideas encountered above, but require you to think a little more. Complete the questions in your exercise book, showing full working for all problems.

Use g = 10 N kg^-1 for all problems.

Task 4