1 of 26© boardworks ltd 2007. 2 of 26© boardworks ltd 2007

1 of 26 © Boardworks Ltd 2007


What happens when a rocket is launched into space?

When the rocket’s engines are fired, chemical energy in the fuel is transferred to kinetic energy in the rocket.

What is work?

This transfer of energy is called work.


In the same way, if a person transfers 250 J of energy, then 250 J of work is done.

This means the units for work are the same as the units for energy – joules.

work done = energy transferred

What is the link between work and energy?

For example, if a person does 500 J of work, then 500 J of energy is transferred.

Work and energy


What factors affect work done?


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 in joules (j).

Force is measured in newtons (N).


Factors affecting work done


A cyclist peddles a bicycle with a force of 1,000 N moving it 250 m.

Calculating work done question 1

= 1,000 x 250

= 250,000 J = 250 kJ

= force x distanceworkdone

How much work has beendone by the cyclist?


A car engine moves a car with a force of 10 kN and does 500 kJ of work. How far has the car travelled?

Calculating work done question 2

= 500,000 / 10,000

work done = force x distance

= 50 m

distance =work done

force


Work, force, distance calculations


Work and energy: true or false?


Powerful cranes


Both these planes are about the same mass but the jet has more powerful engines.

Power is 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 energy faster 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 in joules (J).

work done

time takenpower =

Remember that work done = energy transferred.


A lawnmower engine does 10 kJ of work in 10 seconds. What is the power of the engine?

Calculating power question 1

= 1,000 W = 1 kW

power =work done

time

= 10,000 / 10


A car transfers 12 mJ of energy in 2 minutes. What is the power of the car?

Calculating power question 2

= 100,000 W = 100 kW

= 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 at a 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 gallon – mpg) 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 factors including:

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 is transferred.

watt – The unit of power.

work – The energy transferred to an object by a force and which results in movement.

Glossary


Anagrams


Multiple-choice quiz

1 of 26© boardworks ltd 2007. 2 of 26© boardworks ltd 2007

Documents