Energy and Work

25 October 2011

ObjectivesUnderstand the term Work Done and be able to use the

equation.Know that work done against friction results in heat.

HSW: AF2 – Understanding the applications and implications of science AF5 – Working critically with evidence

Used before in: lesson 1 – resultant forces Will use again in:

PLTS: Team workers – Work collaboratively with others.

Used before in: Science A Will use again in:

KeywordsEnergy, transfer, transform, work, Joule, force, Newton, distance, metre, friction, heat, regenerative, gravity, weight, mass, kilogram.

Outcomes

• ALL MUST be able to describe what work is!

• MOST SHOULD be able to use the equation for work done and describe that work done against friction causes heat.

• SOME SHOULD be able to rearrange the equation for work done and describe how hybrid cars are using otherwise wasted energy.

• Clip

• How can we work out how much energy we burn on a run or a treadmill?

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).

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

Calculating work done question

= 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

= 500,000 / 10,000

work done = force x distance

= 50 m

distance =work done

force

Practical Work

Worksheet

Tasks• After you have done the Practical “Work” ( d’ya get it?)

• 1) copy and complete:• When a force causes a body to move through a

distance, energy is transferred and ……… is done.• Work done = ………. ……………….• work done = …… × distance moved in direction of force• If the direction is up, then the force is weight and the

distance is height: So Work Done = ……… x Height• Weight = ………. x gravity

Work done against frictional forces is mainly transformed into …………..

• 2) Do the summary questions on page 211. (Longman text book)

Work done against friction

• Ray Mears – Youtube• http://www.youtube.com/watch?v=KS9fWAJD_mE • What form of energy is useful because of friction

here? • What form of energy is wasted when we slow

down a car with its brakes? Can we reuse this energy?

• Watch regenerative braking clip – can you describe any benefits of having this type of braking system? http://www.youtube.com/watch?v=2HrQeGzIZoI&feature=related

Work, force, distance calculations