Work and Work and Power NotesPower Notes

What is work ?What is work ?

The product of the force applied to The product of the force applied to an object and the distance through an object and the distance through which that force is applied.which that force is applied.

Is work being done or Is work being done or not?not?

Mowing the lawnMowing the lawn Weight-liftingWeight-lifting Moving furniture up a Moving furniture up a

flight of stairsflight of stairs Pushing against a Pushing against a

locked doorlocked door Swinging a golf clubSwinging a golf club

YESYES YESYES YESYES

NONO

YESYES

Calculating WorkCalculating Work

All or part of the force must All or part of the force must act in the direction of the act in the direction of the movement.movement.

Work TriangleWork TriangleFormulasFormulasW=?W=?

W=F x dW=F x d

F=?F=? F = W / dF = W / d

d=?d=? d = W / Fd = W / F

Do you do more work Do you do more work when you finish a job when you finish a job quickly?quickly? Work does NOT involve time, only force Work does NOT involve time, only force

and distance.and distance. No work is done when you stand in place No work is done when you stand in place

holding an object.holding an object. Labeling work: w = F x dLabeling work: w = F x d Newton X meter (N m)Newton X meter (N m)

Which also = Which also = (kg x m(kg x m22)) ss22

The JouleThe Joule

1 newton-meter 1 newton-meter is a quantity is a quantity known as a known as a joule joule (J).(J).

Named after Named after British physicist British physicist James Prescott James Prescott Joule.Joule.

How quickly work is done.How quickly work is done. Amount of work done per unit time.Amount of work done per unit time. If two people mow two lawns of equal If two people mow two lawns of equal

size and one does the job in half the size and one does the job in half the time, who did more work?time, who did more work?

Same work. Different power exerted.Same work. Different power exerted. POWER = WORK / TIMEPOWER = WORK / TIME

Power TrianglePower TriangleFormulasFormulasP=?P=?

P = W / TP = W / T

W=?W=? W = P x TW = P x T

T=?T=? T = W / PT = W / P

The wattThe watt

A unit named after A unit named after Scottish inventor Scottish inventor James Watt.James Watt.

Invented the steam Invented the steam engine.engine.

P = W/t P = W/t Joules/secondJoules/second 1 watt = 1 J/s1 watt = 1 J/s

wattswatts

Used to measure Used to measure power of light power of light bulbs and small bulbs and small appliancesappliances

An electric bill is An electric bill is measured in measured in kW/hrs.kW/hrs.

1 kilowatt = 1000 W1 kilowatt = 1000 W

Horsepower (hp) = Horsepower (hp) = 745.5 watts745.5 watts

Traditionally associated with engines. Traditionally associated with engines. (car,motorcycle,lawn-mower)(car,motorcycle,lawn-mower)

The term The term horsepower horsepower was developed to was developed to quantify power. A strong horse could quantify power. A strong horse could move a 750 N object one meter in one move a 750 N object one meter in one second.second.

750 N

Work Power

Defined as “Applying a force a

given distance“

Equation

Work = Force x Distance

Newton Meter which is a

Kgm2/s2

Short Name

“JOULE”

Defined as “Rate at which work

gets done”

Equation

Power = Work / Time

In other words

Power = (F x D) ÷ time

Units are Joules per second

“Watts”

US units Horsepower

1HP = 746 Watts