Unlike your everyday experience, where you go to an

after school job and earn money, work in physics does

not pay for your car insurance.

Work has a special meaning in physics.

Work is done on an objectwhen a force is applied to an object and the object is displaced.

Units: Newton x Meter (Nm)

Work is a scalar quantity

Do not confuse applying a force with doing work.

Work is only done if the object moves a distance.

Pushing or pulling on an object may make you sweat but you do no work unless it moves.

W Fd cosOnly the force parallel to the direction

of the displacement does work.

units Nm J Joule:

Work = Force x displacement (cosine of angle for the portion of the force parallel to the distance traveled)

A teacher applies a force to a wall and becomes exhausted.

A book falls off a table and free falls to the ground.

A waiter carries a tray full of meals above his head by one arm across the room. (Careful! This is a tricky question.)

A rocket accelerates through space.

Read the following five statements and Read the following five statements and determine whether or not they represent determine whether or not they represent

examples of work.examples of work.

NO

NO

YES

YES

Power

In Physics Power is the Rate of Doing

Work

PW

t

Fd

t

cos

PowerWork

time

unitsNm

sW Watt:

A watt is a small unitIf you lift a glass of water (~2N) to

your mouth (~0.5 m) in 1 second you do one joule of work and you are doing work at rate of one watt

Since a watt is relatively small, power is often measured in

kilowatts (kW)

Simple Machines

Simple Machines

A simple machine is an object with few moving parts that redirects an

applied force to perform work. Usually a simple machine is

used to reduce the force needed to perform a task.

Simple Machines and Rube Goldberg

Work and Machines• Force of Effort - Fe - The amount of force

that is applied to the machine.

• Force of Resistance - Fr - The amount of force that the machine applies.

• Distance of Effort - de - The distance the force of effort is applied.

• Distance of Resistance - dr - The distance the machine applies the force.

Force of Effort

Force of ResistanceForce of Resistance

Force of Effort

Forc

e o

f R

esi

stan

ce

Distance of E

ffort

Dis

tan

ce o

f R

esi

stan

ce

L e

v e

r

F1 = Fr = Force of Resistance

F2 = Fe = Force of Effort

D1 = dr = Distance of Resistance

D2 = de = Distance of Effort

Fulcrum = Pivot Point

Lever

Lever ClassesFirst Class Lever

Second Class Lever

Third Class Lever

Fulcrum - Resistance - Effort

Effort - Fulcrum - Resistance

Fulcrum - Effort - Resistance

First Class Lever

Pulleys redirect the force of effort.The number of pulleys and their arrangement can reduce the amount of effort force needed to raise a particular load. The number of lifting strands will

determine the advantage to using a pulley system

1 Lifting Strand

2 Lifting Strands2 Lifting Strands

3 Lifting Strands 3 Lifting Strands

Pulleys

How How many many lifting lifting strands strands for each for each pulley pulley system?system?

Wi = Work Input – The Amount of Work applied to the machine

Wo = Work Output – the Amount of Work transferred by the machine

Wi = Fede

Wo = Frdr

Fe=Force of Effort

Fr=Force of Resistance

de=Distance of Effort

dr=Distance of Resistance

Mechanical Advantage = MAF

Fr

e

Ideal Mechanical Advantage = IMAd

de

r

EfficiencyW

W

F d

F dxo

i

r r

e e

100 00

EfficiencyMA

IMAx 100 0

0