Work and Power

WorkImpulse was force over time or how long.How long does not always imply time.It can also mean distance.Work is the effort exerted on something that will

change its energy.If the force is constant and the motion is in a

straight line then:Work = force x distance.More force that needs to be applied the more

work is done.Further the item is moved the more work is done.

Work Applying: If we lift two loads of

gravel up one story we do twice as much work as in lifting one load the same distance.

Why because the force needed to lift the two bags is twice that of the one bag.

If we lift a load two stories instead of one we have also doubled our work as the distance is twice as great.

No movement no workA weight lifter holding a barbell above his head holding it still does no work. Why

He does not move it a certain distance.

If the distance is 0 the work is 0The weigh lifter may feel differently about that

Two categoriesWork done against another force.

Archer stretches her bow string- doing work against the elastic forces of a bow.

Raising something up you raise it up against the force of gravity. – push ups

When you slide something you work against friction.

Work done to change the speed of an object.Bringing automobiles up to speed A golf club hitting a ball

Units of measure for workThe unit of measurement for work combines

a unit of force (N) with a unit of distance (m)Unit of work is the Newton-meter (N*m)Also called a joule (J)One joule of work is done when a force of 1 N

is exerted over a distance of 1 meter. For larger values:Kilojoules (kJ) Thousands of joules or megajoules (MJ) millions of joules

PracticeHow much work is needed to lift a bag of groceries that weighs 200 N to a height of 3 m?

W=FxdW= 200N x 3 m=600 J

PowerBut what about how long?Work says nothing about how long it takes to

do the work.The same about of work it you run a mile if

you do it in 5 min or 30 min.So why are we more tired if we run it in the 5

mins?Power is equal to the amount of work done

per time it takes to do it:Power= work done/time interval

Twice the powerIf you had a car with twice the power you

would not necessarily be able to go twice as fast.

It means the engine can do twice the amount of work in the same time period.

Or the same amount of work in ½ the time.

A powerful engine can get an automobile up to a given speed in less time than the less powerful engine can.

unitsJoules per secondJ/sAlso known as a wattOne watt of power is expended

when 1 joule of work is done in 1 second.

One kilowatt equals 1000 watts (kW)One megawatt (MW) equals 1

million watts.One horse power is ¾ of a kW

QuestionIf a forklift is replaced with a new forklift

that has twice the power, how much more dirt can it lift in the same amount of time?

If it lifts the same dirt, how much faster can it operate?

The fork lift that delivers twice the power will lift twice the load of dirt in the same time or the same load in half the time.