How does work...work? - Peter Bohacek

Transcriber: Andrea McDonough Reviewer: Bedirhan Cinar

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 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.