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  • In Physics,

  • the concepts of work and power help us understand

  • and explain lots of things in our universe.

  • Let's start with work.

  • Positive work is the energy we put into a system,

  • and negative work is energy that is transferred out.

  • Think of positive work as money being added to your bank account,

  • and negative work as money taken out.

  • In the metric system,

  • work and energy are measured in Joules.

  • As an example, let's take a beautiful, old, mechanical grandfather clock.

  • We transfer energy into the clock

  • when we turn the crank

  • to raise the heavy metal cylinders inside the clock.

  • When we do this, we are doing positive work,

  • adding energy to the clock,

  • and that energy is stored as gravitational potential energy.

  • We can calculate the amount of work done by multiplying the force we apply

  • times the distance we over which we apply the force.

  • To raise the metal cylinders,

  • we need to apply a force equal to their weight.

  • That is, equal to the force of gravity

  • pulling downward on the cylinders.

  • These cylinders weight 300 Newtons,

  • which is pretty heavy,

  • about as much as a small child,

  • and if we lift them 1/2 meter,

  • then we do 300 Newtons

  • times 1/2 meter

  • or 150 Joules of work.

  • Power is the rate at which energy is transferred.

  • When we say rate,

  • we mean the amount of energy transferred

  • per unit of time.

  • In the metric system, power is measured in

  • Joules per second,

  • or Watts.

  • The term Watt goes back to James Watt,

  • who came up with the concept of horsepower

  • to measure the amount of power

  • produced by a typical work horse.

  • James Watt was a producer of industrial steam engines,

  • and he wanted his potential customers

  • to be able to make comparisons

  • between his steam engines and a familiar quanity,

  • the power they could get from a working horse.

  • It was such a useful idea

  • that the metric system unit for power, the Watt,

  • is named after James Watt.

  • Following in James Watt's footsteps,

  • let's compare the amount of power it takes

  • to run this grandfather clock

  • to the power we'd need to run

  • a bright, 100-Watt light bulb.

  • We can measure the power a person uses

  • to wind the clock

  • by dividing the amount of work they did

  • by the time it took them to do it.

  • If it takes 1 minute, or 60 seconds,

  • to lift the weights,

  • then they are doing 150 Joules

  • divided by 60 seconds,

  • or 2.5 Joules per second of work.

  • They are adding energy to the clock

  • in the rate of 2.5 Watts.

  • You would need about 40 times as much

  • to run a bright, 100-Watt light bulb.

  • Before we let the clock run,

  • the energy is stored

  • as gravitational potential energy of the cylinders.

  • It's like your bank account

  • when you have just deposited money.

  • But if we let the clock run,

  • the cylinders slowly move downward.

  • Energy is leaving the clock.

  • In fact, when the cylinders get to the bottom,

  • all the energy that we put in will have left.

  • So how much power does the clock use?

  • That is, how many Joules of energy per second leave the clock

  • if it takes 5 days for the cylinders to return to their original position?

  • We can figure this out

  • because we already know how much work we did

  • when we lifted the cylinders:

  • 150 Joules.

  • But this time, it took 5 days rather than a minute.

  • Five days is 5 times 24

  • times 60

  • times 60 again

  • or 432,000 seconds.

  • So we divide the work done by the time

  • and find the answer of about 0.00035 Joules per second,

  • or about 0.35 milliwatts.

  • That's a tiny amount of power.

  • This clock uses so little power

  • that you could run almost 300,000 clocks

  • using the same power it takes to run one 100-Watt light bulb.

  • That's right, you could run a clock in every house

  • in a medium sized city with that much power.

  • That's a pretty amazing conclusion

  • and it took knowledge of work

  • and power to figure it out.

In Physics,

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B1 中級

TED-ED】仕事って...どうやってするの?- ピーター・ボハチェク (【TED-Ed】How does work...work? - Peter Bohacek)

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    稲葉白兎 に公開 2021 年 01 月 14 日
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