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Year 10
OCR
Higher

Power calculations (P = W/t)

I can describe power and calculate the rate at which energy is transferred by a force.

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New
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Year 10
OCR
Higher

Power calculations (P = W/t)

I can describe power and calculate the rate at which energy is transferred by a force.

Download all resources
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Share activities with pupils
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Lesson details

Key learning points

  1. Power is equal to the rate at which work is done.
  2. Power is equal to the rate at which energy is transferred.
  3. Power = work done ÷ time.
  4. Power is measured in watts (W).

Keywords

  • Work - Work is done whenever a force makes an object move. The amount of work is force multiplied by distance moved in the direction of the force.

  • Power - Power is the rate at which work is done.

  • Rate - The rate at which something changes is usually equal to the amount of change each second.

  • Watt (W) - The unit for measuring power is the watt (W).

  • Kilowatt (kW) - A thousand watts is called a kilowatt (kW).

Common misconception

Pupils often mix up the terms power and force, which are often used interchangeably in everyday speech.

Provide opportunities for pupils to describe events in which the terms need to be used appropriately, for example in lifting two different sized weights (different forces) whilst increasing their energy at the same rate (same power).

This lesson provides a good opportunity for pupils to practise measuring power by moving up the stairs, step-ups onto a low platform or lifting objects of different weights (avoids pupils using their own weight) in order to develop a better understanding of the numbers in power calculations.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
Which of these units can be used to measure energy?
kilogram
Correct answer: kilojoule
kilometre
kilowatt
Q2.
Which of these is the unit of work done?
gram
Correct answer: joule
newton
watt
Q3.
Which of these is the unit of force?
joule
kilogram
metre
Correct answer: newton
Q4.
What is the equation for work done?
Correct answer: work done = force × distance
work done = force × time
work done = force ÷ distance
work done = force ÷ time
Q5.
work done = × distance
Correct Answer: force, F
Q6.
Put these in order of decreasing work done, starting with the greatest amount of work done.
1 - A boat is pulled 5 m with a force of 21 N.
2 - A 51 N load is lifted 2 m.
3 - A trolley is pushed 6 m with a force of 15 N.
4 - A crate is pushed 2 m with a force of 40 N.

Exit quiz

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6 Questions

Q1.
Aisha does 6000 J of work when she runs up a hill. How much work does she do when she runs up the same hill in less time?
Less than 6000 J
Correct answer: 6000 J
More than 6000 J
Q2.
Match the quantities to their units.
Correct Answer:force ,newtons (N)

newtons (N)

Correct Answer:power ,watts (W)

watts (W)

Correct Answer:speed,metres per second (m/s)

metres per second (m/s)

Correct Answer:work done ,joules (J)

joules (J)

Q3.
Aisha’s power when she runs up a hill is 400 W. What is her power when she runs up the same hill in less time?
Less than 400 W
400 W
Correct answer: More than 400 W
Q4.
Which of these is the correct equation for power?
power = work done × distance
power = work done ÷ distance
power = work done × time
Correct answer: power = work done ÷ time
Q5.
What would Aisha’s power be if she walked up the hill 4× more slowly, compared to when she ran up it?
4× greater
2× greater
the same
2× smaller
Correct answer: 4× smaller
Q6.
Put these in order of decreasing power, starting with the greatest amount of power.
1 - A crate is pushed 2 m with a force of 40 N in 3 seconds
2 - A 50 N load is lifted 2 m in 5 seconds
3 - A trolley is pushed 6 m with a force of 15 N in 6 seconds
4 - A boat is pulled 5 m with a force of 20 N in 10 seconds