Work and Work and machinesmachines

Making jobs easierMaking jobs easier

WorkWork Work is done on an Work is done on an object when a force is object when a force is exerted on an object exerted on an object that causes the object that causes the object to move some distance.to move some distance.

No work without motionNo work without motionNo work without force in No work without force in the same directionthe same direction

Motion and forceMotion and force Person does not Person does not

work on weights work on weights – weights do not – weights do not movemove

Person does Person does work on weights work on weights – weights moved– weights moved

Force on weights

Force on weights

Summer Olympics 2000 Sydney Australia By Lou Jones

http://www.pricespower.com

Direction and forceDirection and force Horse does not Horse does not

work on the rider – work on the rider – force not in same force not in same directiondirection

Horse does work Horse does work on the cart - force on the cart - force and motion in the and motion in the same directionsame direction

Force on person

motion motion

Force on cart

Calculating WorkCalculating WorkWork = Force X DistanceWork = Force X Distance

Force units = Newton, NForce units = Newton, NDistance units = meter, mDistance units = meter, mWork units = Joule, JWork units = Joule, J

J = N*mJ = N*m

The unit of work is the The unit of work is the JouleJoule

Force x Distance = Work1 Newton x 1 meter = 1 Newton meter

1 Newton meter = 1 Joule

Example 1:Example 1: A high jumper weighs 700 newtons. What work does the A high jumper weighs 700 newtons. What work does the

jumper perform in jumping over a bar 2.0 meters high?jumper perform in jumping over a bar 2.0 meters high? Answer: W = F x dAnswer: W = F x d W = 700N x 2.0 m W = 700N x 2.0 m = 1400 nm= 1400 nm = 1400 Joules= 1400 Joules

No work is done if there No work is done if there is no distance!is no distance!

The statue of liberty has been holding up her torch for an awfully long time. How much work has she done?

Answer: 0! Although it takes a force to hold the torch against the force of gravity, there is no motion so no work is done.

Making work easierMaking work easier MachinesMachines

Change the amount of force you exertChange the amount of force you exert Change the distance over which you Change the distance over which you

exert forceexert force Change the direction in which you exert Change the direction in which you exert

forceforce Do change the amount of work doneDo change the amount of work done

Total work does not Total work does not changechange

Work = Force X DistanceWork = Force X Distance If a machine allows you to apply a If a machine allows you to apply a

smaller force the distance will smaller force the distance will increaseincrease

If a machine reduces the distance If a machine reduces the distance the force will increasethe force will increase

If a machine allows you to apply a If a machine allows you to apply a force in a different direction it force in a different direction it doesn’t change how much force you doesn’t change how much force you have to apply or how far you move ithave to apply or how far you move it

Mechanical advantageMechanical advantage The number of times a force

exerted on a machine is multiplied by the machine

Mechanical ad. = Output force Input force If M.A. > 1 machine increases

force needed but distance decreases

If M.A. < 1 machine reduces force needed but distance increases

EfficiencyEfficiency Efficiency compares the output Efficiency compares the output

work to the input work (%)work to the input work (%) Cannot be greater than 100%Cannot be greater than 100%

Efficiency usually < 100%Efficiency usually < 100% FrictionFriction

Types of simple Types of simple machinesmachines

Inclined planeInclined plane WedgeWedge ScrewScrew LeverLever Wheel and Wheel and

axleaxle pulleypulley

Image taken from:http://www.daniel-wright.district103.k12.il.us/ccheifetz/physics.html

Inclined planeInclined plane

teacher.scholastic.com/dirtrep/ simple/img/plane.gif

WedgeWedge

teacher.scholastic.com/dirtrep/ simple/img/wedge.gif

ScrewScrew

teacher.scholastic.com/dirtrep/ simple/img/screw.gif

LeverLever

teacher.scholastic.com/dirtrep/ simple/img/lever.gif

Wheel and axleWheel and axle

teacher.scholastic.com/dirtrep/ simple/img/wheel.gif

PulleyPulley

                                                       

teacher.scholastic.com/dirtrep/ simple/img/pulley.gif