Work and Power

What does work mean to you?

In physics, work has a very specific meaning. It is the action to make things change

Work= force x distance W=F x d

Unit: N-m : Joule

Work

W = F x d

Therefore:

You must have MOTION in order to have work… something has to change position

Work = Force x distance

Work …the motion has to take place parallel to the

direction of the force

Garcon does work whenhe picks up the trayGarcon does work whenhe picks up the tray

but not while hecarries it around

the room

Garcon is not doing work

Work and Energy…the motion (distance) is parallel to the

direction of the force

Is Atlas doing work?

NONO

Atlas is staying in place

Atlas is not doing work

Is work being done on the wall?

work may be done on the prisoner’s muscles …stretching,

squeezing them, pumping the blood through the veins..

but this is not work being done on the wall be clear where the

work goes

Is work being done?

Example Problems

How much work is done by a toy car with an applied force of 12 N which is moved for 39 m?

Power

Pit Pony

A little bit of history

To find out how the power of his engine compared to that of a horse, James Watt measured how fast an average horse could do work. He found the answer and expressed the amount of work performed per second as a horsepower.

1 horsepower = 746 WattsA horse exerting 1 horsepower can raise 330

pounds of coal 100 feet in a minute.In the US we rate engines in terms of

horsepower (hP) and electricity in watts (kilowatts, megawatts)

Horsepower

PowerPower – the rate at which work is done.

Power = Work/time

Unit is Watt (joule/second)

ONE OF 2 THINGS

POWER = WORK DONE TIME

2X WORK OR WORK TIME ½ TIME

What can a more powerful engine do?

Example ProblemsIf a 450 N boulder is dropped 2.4 m in 25 seconds, how much power is exerted on the object?

The weight lifter used a force of 980 N to raise the barbell over her head in 5.21 seconds. Approximately how much work did she do in raising the barbell?

A mechanic used a hydraulic lift to raise a 12,054 N car 1.89 m above the floor of a garage. It took 4.75 s to raise the car. What was the power output of the lift?

Example ProblemsExample Problems