Work and MachinesWork and Machines

Chapter 5 Section 1Chapter 5 Section 1

What is What is workwork??

Work is the transfer of energy that occurs when a force makes an object move

The amount of The amount of workwork is dependent is dependent onon

a. a. the the forceforce exerted exertedb. b. the the distancedistance over which the over which the force is force is

appliedapplied

Work = Force X distanceWork = Force X distance

Two conditions that must be met for Two conditions that must be met for workwork to occur to occur

An object MUST MOVE!An object MUST MOVE!Explain why this is true.Explain why this is true.

The The motion of the objectmotion of the object must be in the must be in the same direction as the applied same direction as the applied forceforce on the on the objectobject

How are How are work work and and energyenergy related? related?

1. 1. Energy is always transferredEnergy is always transferred. . . . . . a. a. fromfrom the object doing the the object doing the work work b. b. toto the object on which the the object on which the workwork is is

donedone..2. Both 2. Both work & energy are measured in work & energy are measured in

JoulesJoules, the SI unit of work & energy, the SI unit of work & energy3. The unit 3. The unit JouleJoule was named after an was named after an

EnglishEnglish physicist, James Prescott Joule.physicist, James Prescott Joule.44. . One Joule is equal to One Joule is equal to

a Newton-meter (Na Newton-meter (N●●m)m)

Calculating WorkCalculating Work

Work equals force (in Newtons) times Work equals force (in Newtons) times distancedistance

w= f x dw= f x d

F D

w

The brakes on a bicycle apply The brakes on a bicycle apply 125 N125 N of of frictional force to the wheels as the frictional force to the wheels as the bicycle travels bicycle travels 14.0 m14.0 m. How much . How much workwork have the brakes done on the bicycle?have the brakes done on the bicycle?

Practice ProblemsPractice Problems

F = 125 Nd = 14.0 mW = ?

W = F x d

W = (125 N)(14.0 m) W = 1,750 J

A hydraulic lift performs A hydraulic lift performs 12,500 J12,500 J of work raising a car of work raising a car 0.5 m0.5 m. How . How much much forceforce has the lift applied to has the lift applied to

the car?the car?

Practice ProblemsPractice Problems

W = 12,500 Jd = 0.5 mF = ?

F = W / d

F = 12,500 J / 0.5 m F = 25,000 J

POWERPOWER

1. 1. The amount of The amount of workwork done in a certain done in a certain amount of time (rate at which work is done).amount of time (rate at which work is done).

2. 2. Power is measured in Power is measured in Watts (W)Watts (W), named , named after the Scottish inventor James Watts.after the Scottish inventor James Watts.

Calculating Calculating PowerPower

Power equals work divided by Power equals work divided by timetime

P=w/TP=w/TPower is measure in watts (W)Power is measure in watts (W)

Power TrianglePower Triangle

P t

w

For exampleFor example

The number of Watts marked on light The number of Watts marked on light bulbs tells you how much work the bulb bulbs tells you how much work the bulb does in a period of time - that is power. does in a period of time - that is power.

Do you see how work and power and Do you see how work and power and related?related?

Practice Practice ProblemsProblems

While rowing in a race, John does While rowing in a race, John does 3960 J3960 J of work on the oars in of work on the oars in 60.0 s60.0 s. . What is his What is his powerpower output in watts? output in watts?

W = 3960 JW = 3960 J P = W / tP = W / tt = 60.0 st = 60.0 sP = ?P = ?

P = 3960 J / 60.0 sP = 3960 J / 60.0 s P = 66 W P = 66 W

While carrying a box of books for While carrying a box of books for 20.0 s20.0 s, your power output is , your power output is 720 W720 W. . How much How much workwork have you done? have you done?

P = 720 WP = 720 W W = PtW = Ptt = 20.0 st = 20.0 sW = ?W = ?

W = (720 J)(20.0 s)W = (720 J)(20.0 s) W = 14,400 J W = 14,400 J

Practice ProblemsPractice Problems

Chapter 5Chapter 5

Section 2Section 2

MachinesMachines

Transfer mechanical energy Transfer mechanical energy from one object to anotherfrom one object to another

Make work Make work seemseem easier easier (cannot reduce (cannot reduce the the amountamount of work done) of work done)

Can Can multiply forcemultiply force (wrench removing(wrench removing bolt),bolt), change the directionchange the direction of a forceof a force (flagpole)(flagpole) or or change the distancechange the distance over over which a force is appliedwhich a force is applied (loading ramp)(loading ramp)

Two types of forcesTwo types of forces are are involved in machines:involved in machines:

INPUT FORCEINPUT FORCE – –

force put in; also force put in; also

called the called the efforteffort forceforce

OUTPUT FORCEOUTPUT FORCE – force out of – force out of machine; also called the machine; also called the resistanceresistance force force

The same amount of work can be The same amount of work can be done bydone by::

1. Applying a small amount of force over a long 1. Applying a small amount of force over a long distancedistance

OROR 2. By applying a large force over a short 2. By applying a large force over a short

distancedistance

Increasing distance reduces the Increasing distance reduces the amount of force neededamount of force needed to do the to do the workwork

Machines help move things Machines help move things that resist being movedthat resist being moved

Ex: Dolly used to move a Ex: Dolly used to move a heavy refrigeratorheavy refrigerator

Mechanical AdvantageMechanical Advantage

Measures how much a machine Measures how much a machine multiplies force or distancemultiplies force or distanceMA>1: multiplies the effort forceMA>1: multiplies the effort forceMA<1: increases distance or speedMA<1: increases distance or speed

MA = MA = output forceoutput force = = input distanceinput distance input force input force output distance output distance

Has no units! (they cancel out)Has no units! (they cancel out)

An ideal machine with no An ideal machine with no friction would have the same friction would have the same input work and output work, input work and output work, however some energy however some energy transferred is change to heat transferred is change to heat due to frictiondue to friction

EFFICIENCYEFFICIENCY How much useful work a How much useful work a machine machine

can do (expressed as %).can do (expressed as %).

Some work always overcomes friction, Some work always overcomes friction, therefore some energy is always lost as therefore some energy is always lost as heat! heat! This means %eff < 100% !!This means %eff < 100% !!

% efficiency = % efficiency = work outputwork output x 100 x 100

work inputwork input

% = W% = WOO / W / WII x 100 x 100

The efficiency of a machine The efficiency of a machine is always less than 100 %is always less than 100 %

Why?Why?Friction plays a big role in Friction plays a big role in

the output of a machinethe output of a machine