4. Work and Power v1.0

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What happens when a rocket is

launched into space?

When the rockets engines are

fired, chemical energy in the fuel

is transferredto kinetic energy inthe rocket.

What is work?

This transfer of energy is called work.


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In the same way, if a person transfers 250J of energy,

then 250J of work is done.

This means the units for work are the same as the units

for energyjoules.

work done = energy transferred

What is the link between work and energy?

For example, if a person does 500J of work, then 500J

of energy is transferred.

Work and energy


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What factors affect work done?


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How is work calculated?

The work done on an object can be calculated using this

equation:

work done = force x distance moved

Distance moved is measured in metres (m).

Work done is measured injoules (j).

Force is measured in newtons (N).


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Factors affecting work done


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A cyclist peddles a

bicycle with a force of

1,000N moving it 250m.

Calculating work done question 1

= 1,000 x 250

= 250,000J = 250kJ

= force x distancework

done

How much work has been

done by the cyclist?


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A car engine moves a car

with a force of 10kN and

does 500kJ of work. How

far has the car travelled?

Calculating work done question 2

= 500,000 / 10,000

work done = force x distance

= 50m

distance =

work done

force



Work, force, distance calculations


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Powerful cranes



Both these planes are about

the same mass but the jet

has more powerful engines.

Poweris the rate at which work is done. A more powerful

engine therefore does work (i.e. transfers energy) faster

than a less powerful engine.

Which is most powerful?

This means that chemical

energy in the fuel can be

transferred to kinetic energyfaster so it will have greater

acceleration and a higher top

speed.



How is power calculated?

The power exerted by an object can be calculated using one

of two equations:

Time is measured in seconds (s).

Power is measured in watts (W).

Work done is measured injoules (J).

work done

time takenpower =

Remember that work done = energy transferred.



A lawnmower engine

does 10kJ of work in

10 seconds. What is

the power of the

engine?

Calculating power question 1

= 1,000W = 1kW

power =work done

time

= 10,000 / 10



A car transfers 12mJ of

energy in 2 minutes.

What is the power of

the car?

Calculating power question 2

= 100,000W = 100kW

= 12,000,000 / 120

power =work done

time=

energy transfer

time



Power, work and time calculations



More power!

What advantages and disadvantages are there in using a

more powerful machine?



Which plane will consume fuel the fastest?

The jet plane will have a higher

fuel consumption than the

propeller plane because its

engines are more powerful.

How is power related to fuel consumption?

A higher-powered engine will

consume fuel more quickly

than a lower-powered engine

because it transfers energy ata faster rate. However, more

work will be done in the same

time.



Why is fuel consumption important?

In what situations might fuel consumption be an important

consideration?

When buying a car, its fuel

consumption (measured in

miles per gallonmpg) is an

important factor to think about.

This is because cars with a

higher fuel consumption are:

more expensive to run due to increased need to refuel,

as well as higher road taxes

more polluting due to higher carbon dioxide emissions

the higher the emissions, the greater the road tax.

What factors might affect the fuel consumption of a car?



These figures are, however, based on ideal driving conditions

and are unlikely to be achieved in real-world driving.

The fuel consumption of a specific car will vary with factorsincluding:

All car manufacturers publish data on the fuel consumption on

their cars, usually for two different speeds (e.g. urban and

motorway).

Factors affecting fuel consumption

its speed, and the amount of harsh acceleration and

braking

the number of passengers and amount of luggage it is

carrying

the quality of the road surface.



fuel consumption The amount of fuel used by an

engine, and which increases with more powerful engines.

joule The unit of work and energy.

power The rate at which work is done, or energy istransferred.

watt The unit of power.

work

The energy transferred to an object by a force andwhich results in movement.

Glossary



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4. Work and Power v1.0