Chapter 5Chapter 5Work and MachinesWork and Machines

WORKWORK The transfer of The transfer of

energy that occurs energy that occurs when a force makes when a force makes an object movean object move

In order for In order for workwork to to be done a force must be done a force must make an object make an object movemove

2 Conditions of work2 Conditions of work

1. The object 1. The object must movemust move

2. Movement must be in the 2. Movement must be in the same directionsame direction as as the force appliedthe force applied

Work and EnergyWork and Energy

When work is done there is always a transfer When work is done there is always a transfer of energyof energy

Energy is the ability to do workEnergy is the ability to do work

When you do work on an object, you increase When you do work on an object, you increase its energyits energy

Calculating workCalculating work

Work (J) = Force (N) x distance (m)Work (J) = Force (N) x distance (m)

W = FdW = Fd

Example: Example: How much work is done when a force of 33N pulls a wagon 13 meters?W = (33N) x (13m) = 429 J

Work ExamplesWork Examples

A couch is pushed with a force of A couch is pushed with a force of 75 N and moves a distance of 5 m 75 N and moves a distance of 5 m across the floor. How much work across the floor. How much work

is done in moving the couch?is done in moving the couch?

Work ExamplesWork Examples

W = f x dW = f x dW = 75 N x 5 mW = 75 N x 5 m

W = W = 375 J375 J

Work ExamplesWork Examples

The brakes on a car do 240,000 J The brakes on a car do 240,000 J of work in stopping the car. If the of work in stopping the car. If the

car travels a distance of 50m car travels a distance of 50m while the brakes are being while the brakes are being

applied, what is the force the applied, what is the force the brakes exert on the car?brakes exert on the car?

Work ExamplesWork Examples

W = f x dW = f x d240,000 J = f x 50m240,000 J = f x 50m

f = f = 4800 N4800 N

PowerPower

The rate at which work is doneThe rate at which work is done

SI unit for power is the SI unit for power is the Watt (W)Watt (W)

P (watts) = Work (J)/Time (s)P (watts) = Work (J)/Time (s)

Usually expressed in kilowattsUsually expressed in kilowatts

Power ExamplesPower Examples

In lifting a baby from a crib, 50J In lifting a baby from a crib, 50J of work are done. How much of work are done. How much power is needed if the baby is power is needed if the baby is

lifted in 2.0 s?lifted in 2.0 s?

Power ExamplesPower Examples

P = w/tP = w/tP = 50 J/ 2.0 sP = 50 J/ 2.0 s

P = P = 25 W25 W

Power ExamplesPower Examples

The power produced by an The power produced by an electric motor is 500 W. How electric motor is 500 W. How

long will it take the motor to do long will it take the motor to do 10,000 J of work?10,000 J of work?

Power ExamplesPower Examples

P = w/tP = w/t500 W = 10,000 J/ t500 W = 10,000 J/ t

t = t = 20 s20 s

MachineMachine

A device that makes doing work easierA device that makes doing work easier

2-Forces Involved in 2-Forces Involved in Work Done by MachinesWork Done by Machines

11. . Input Force:Input Force: The force that is applied to the The force that is applied to the machine. (effort force)machine. (effort force)

Given the symbol Given the symbol FFinin

2.2. Output Force:Output Force: The force applied by the The force applied by the machine.machine.

Given the symbol Given the symbol FFoutout

Mechanical AdvantageMechanical Advantage

The ratio of the output force to the input force.The ratio of the output force to the input force.

Can be calculated using the following equation.Can be calculated using the following equation.

Calculating Mechanical Calculating Mechanical AdvantageAdvantage

Calculate the mechanical advantage Calculate the mechanical advantage of a hammer if the input force is 125 of a hammer if the input force is 125 N and the output force is 2,000 N.N and the output force is 2,000 N.

Calculating Mechanical Calculating Mechanical AdvantageAdvantage

MA = FMA = Foutout/F/Finin

MA = 2,000 N / 125 NMA = 2,000 N / 125 N = = 16 16

Conserving EnergyConserving Energy

A machine A machine cannotcannot create energy, create energy, so…so…

WWoutout can can never benever be greater than greater than WWinin

Which means that…Which means that…

WWoutout will always be will always be lessless that that WWinin ……

because friction changes some of the because friction changes some of the energy to heat.energy to heat.

Ideal MachineIdeal Machine

A machine without friction.A machine without friction.

WWinin = W = Woutout

EfficiencyEfficiency

A measure of how much of the input work put into a A measure of how much of the input work put into a machine is change into useful output.machine is change into useful output.

Efficiency Cont…Efficiency Cont…

• The efficiency of an ideal machine is 100 percent.

• The efficiency of a real machine is always less than 100 percent.

How can machines be made How can machines be made more efficient???more efficient???

• by adding a lubricant, such as oil or grease, to surfaces that rub together.

Simple MachinesSimple Machines

A Machine that does A Machine that does work with only work with only one one movementmovement of the of the machinemachine

6 types6 types of simple of simple machines: lever, pulley, machines: lever, pulley, wheel and axle, inclined wheel and axle, inclined plane, screw, and wedgeplane, screw, and wedge

LeverLever A bar that is free A bar that is free

to pivot or turn to pivot or turn around a fixed around a fixed pointpoint

The fixed point is The fixed point is called a fulcrumcalled a fulcrum

There are 3 There are 3 classes of leversclasses of levers

Ex. – Screwdriver opening a paint can

Ex. – Wheelbarrow

Ex. – Baseball Bat

PulleyPulley

A grooved wheel with a A grooved wheel with a rope, chain, or cable rope, chain, or cable running along the running along the groovegroove

The axle of the pulley The axle of the pulley acts as the fulcrumacts as the fulcrum

Fixed and movableFixed and movable Example - ElevatorsExample - Elevators

Wheel and AxleWheel and Axle

consists of a shaft or consists of a shaft or axle attached to a larger axle attached to a larger wheel.wheel.

The wheel and axle The wheel and axle rotate togetherrotate together

Examples:Examples: Pencil Pencil sharpeners, door knobs, sharpeners, door knobs, screw drivers, faucet screw drivers, faucet handleshandles

Inclined PlaneInclined Plane

A sloping surface, such A sloping surface, such as a ramp.as a ramp.

Reduces the amount of Reduces the amount of force required to do force required to do workwork

ScrewScrew

An inclined planeAn inclined plane wrapped in a spiral wrapped in a spiral around a cylindrical around a cylindrical postpost

Example – Lid of a Example – Lid of a peanut butter jarpeanut butter jar

WedgeWedge

An An inclined planeinclined plane with with one or two sloping sidesone or two sloping sides

It It changes the directionchanges the direction of theof the input forces input forces

Example - KnifeExample - Knife

Compound MachineCompound Machine

Two or moreTwo or more simple machines that operate simple machines that operate togethertogether

Examples – car, can openerExamples – car, can opener

Compound MachinesCompound Machines