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Warm Up 9/25

• List the following letters in order from most GPE to lowest KE.

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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.

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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

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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 meters (m).

Work done is measured in joules (j).

Force is measured in newtons (N).

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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?

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

Calculating work done question 2

= 500,000 / 10,000

work done = force x distance

= 50 m

distance =work done

force

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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.

What is Power?

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.

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How is power calculated?

The power exerted by an object can be calculated using the following equation:

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.

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A lawnmower engine does 10,000 J of work in 10 seconds. What is the power of the engine?

Calculating power question 1

= 1,000 W

power =work done

time

= 10,000 / 10

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A car transfers

1.2 x 107 J 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

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Work and Power Song

• http://www.youtube.com/watch?v=5EsMmdaYClQ

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More power!

What advantages and disadvantages are there in using a more powerful machine?

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Powerful cranes

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Work, force and distance calculations

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Factors affecting work done