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Power

Power. The rate at which work is done. Work. A 60.0kg jogger runs up a long flight of stairs in 4.00 s. The vertical height of the stairs is 4.50m. Another 60.0kg person walks up the same stairs. It takes 10.0s. a) How much work did the jogger do? b) How much work did the walker do?

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Power

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  1. Power The rate at which work is done

  2. Work • A 60.0kg jogger runs up a long flight of stairs in 4.00 s. The vertical height of the stairs is 4.50m. Another 60.0kg person walks up the same stairs. It takes 10.0s. • a) How much work did the jogger do? • b) How much work did the walker do? • They both did the same work. W = Fdcosϑ W = (60)(9.8)(4.5)cos0o W = 2650 J

  3. Power • Power is the rate at which work is done. It is the amount of energy transformed over time. • In the previous problem, the jogger did the same work faster. The jogger is therefore more powerful than the runner. • P = W/t • Power is measured in Watts (W). • Jogger: P = 2650 J/4.00s P = 660.W • Runner: P = 2650 J/10.0s P = 265W

  4. Power • Sometimes it is convenient to describe Power indicating the net force applied to an object and its speed. • P = W/t • P = Fd/t • P = Fv

  5. Power • If a car generates 18hp when travelling at a steady 88km/h, what must be the average force exerted on the car due to friction and air resistance?

  6. Power • How much work can a 3.0hp motor do in 1.0 hour?

  7. Power • A person is pulling a sled that has a mass of 93 kg. The sled starts from rest. There is no friction. The person is pulling with a horizontal force of 225 N over a distance of 12.0 m. What was the average power developed by the person?

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