Work and Power

1. Work and Power

2. Work • Impulse 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.

3. 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.

4. No movement no work • A 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 0 • The weigh lifter may feel differently about that 

5. Two categories • Work 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

6. Units of measure for work • The 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

7. Practice • How much work is needed to lift a bag of groceries that weighs 200 N to a height of 3 m? • W=Fxd • W= 200N x 3 m= • 600 J

8. Power • But 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

9. Twice the power • If 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.

10. units • Joules per second • J/s • Also known as a watt • One 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

11. Question • If 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.