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• In Physics,

• the concepts of work and power help us understand

• and explain lots of things in our universe.

• 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

• 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 TED-Ed watt clock energy power metric system

# 【TED-Ed】How does work...work? - Peter Bohacek

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稲葉白兎 posted on 2015/01/20
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