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Power

Power. Power is the rate at which work is done. Power is how fast work is done. What are the factors of power?. Power formula:. WORK. POWER =. TIME. Power unit. As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time.

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Power

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  1. Power Power is the rate at which work is done. Power is how fast work is done. What are the factors of power?

  2. Power formula: WORK POWER = TIME

  3. Power unit • As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time. • Thus, a unit of power is joules/second. • To honor him (who?), the standard metric unit of power is also called the watt. (Who is the watt named after?)

  4. Power Ratings • Most machines are designed and built to do work on objects. • All machines are typically described by a power rating. • The power rating indicates the rate at which that machine can do work upon other objects. • Thus, the power of a machine is the work/time ratio for that particular machine.

  5. Horsepower • For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. • One horsepower is equivalent to approximately 750 watts. • Note: Horsepower is NOT a metric unit!

  6. Power, Work, Time relationship • The power equation suggests that a more powerful engine can do the same amount of work in less time. • Power = Work Time

  7. A person is also a machine which has a power rating. • Some people are more powerful than others; that is, they are capable of doing the same amount of work in less time OR more work in the same amount of time.

  8. A Power Problem • 1. Jack and Jill ran up the hill. Jack is twice as massive as Jill; yet Jill ascended the same distance in half the time. Who did the most work? Who delivered the most power? Explain your answers.

  9. Jack and Jill answer • Let’s suppose Jack is 60 kg (= ___ N?) • Let’s suppose Jill is 30 kg (= ___ N?) • Let’s suppose the hill is 10 m high. • WJack = F x d = • WJill = F x d = • Who did the most work?

  10. Jack and Jill answer, continued • Suppose Jack’s time is 20 seconds and Jill’s time is 10 seconds. • So, PowerJack = W/t = • And, Power Jill = W/t = • Who is the more powerful?

  11. Another Power Problem • 2. If little Nellie Newton lifts her 40-kg body a distance of 0.25 meters in 2 seconds, then what is the power delivered by little Nellie's biceps?

  12. Little Nellie Newton answer • Nellie’s mass is 40 kg (= ___ N) • P = W = F x d t t • P = (400 N) (0.25 m) 2 sec • P = 50 Nm/s = 50 J/s = 50 watts = 50 W

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