Chapter 9Chapter 9

Work and energyWork and energy

WorkWork

What is work?What is work? To a scientist work is done when changing To a scientist work is done when changing

motionmotion Work is force applied multiplied by the Work is force applied multiplied by the

distance the force actsdistance the force acts W = Force x DistanceW = Force x Distance W = F x dW = F x d Only if the force and the direction are the Only if the force and the direction are the

samesame

WorkWork

When an Olympic weight lifter presses a When an Olympic weight lifter presses a barbell over his headbarbell over his head

He is doing workHe is doing work he must hold it there until the judges say he must hold it there until the judges say

he can put it downhe can put it down He is not doing workHe is not doing work Big force but no distanceBig force but no distance

Units of workUnits of work

W = F x dW = F x d Netwons x meters = NNetwons x meters = N··mm Or kgOr kg·m/s·m/s Or Joules = JOr Joules = J An apple weighs about 1 NAn apple weighs about 1 N Lift it one meterLift it one meter That is 1 That is 1 NN··m of work or 1 J of workm of work or 1 J of work

Calculating WorkCalculating Work

Use the equation W = F x dUse the equation W = F x d How much work does it take to lift a 200 N How much work does it take to lift a 200 N

weight 2 m off the floor?weight 2 m off the floor? How much work does it take to hold a 200 How much work does it take to hold a 200

N weight 2 m off the floor?N weight 2 m off the floor? How much work is done if you drop a 2.5 How much work is done if you drop a 2.5

N book 3 meters?N book 3 meters? What does the work?What does the work?

PowerPower

Running up stairs is harder than walking Running up stairs is harder than walking up stairsup stairs

Why? They both do the same amount of Why? They both do the same amount of work.work.

Running does the same work more quicklyRunning does the same work more quickly Power is the rate at which work is done.Power is the rate at which work is done. Power = Power = WorkWork

Time Time

PowerPower

Measured in units called watts (W)Measured in units called watts (W) 1 watt is the power to do 1 J of work in 1 s1 watt is the power to do 1 J of work in 1 s W = W = J. J.

s s A student lifts a 12 N textbook 1.5m of the A student lifts a 12 N textbook 1.5m of the

floor in 1.5 s. How much work did he do?floor in 1.5 s. How much work did he do? How much power did he use?How much power did he use?

PowerPower

A 43 N force is exerted through 2.0 m A 43 N force is exerted through 2.0 m distance for 3.0 s. How much work was distance for 3.0 s. How much work was done?done?

How much power was used?How much power was used?

MachineMachine

Machines make work easier.Machines make work easier. They multiply force or change its directionThey multiply force or change its direction They multiply force by using a small force They multiply force by using a small force

to go a long distanceto go a long distance Things like ramps, levers, etc. Things like ramps, levers, etc.

1 m

75 N

W = 75 N x 1 m = 75 J

W = 25 N x 3 m = 75 J

3 m

25 N

Mechanical AdvantageMechanical Advantage

How many time a machine multiplies the How many time a machine multiplies the input forceinput force

Mechanical Advantage = Mechanical Advantage = output force output force input force input force

Mechanical Advantage = Mechanical Advantage = input distanceinput distance output distance output distance

Mechanical advantage greater than 1 Mechanical advantage greater than 1 multiples forcemultiples force

Less than 1 it multiples distance, less forceLess than 1 it multiples distance, less force

EnergyEnergy

Energy is the ability to do workEnergy is the ability to do work Whenever you do work you transfer Whenever you do work you transfer

energy from one thing to anotherenergy from one thing to another It can only be observed when it is It can only be observed when it is

transferredtransferred Measured in the same units as work- Measured in the same units as work-

joulesjoules

Potential energyPotential energy

Stored energyStored energy Energy of positionEnergy of position Stretched rubber bandStretched rubber band Gravitational potential energy – any time Gravitational potential energy – any time

gravity supplies the forcegravity supplies the force Most often because it is raised off the Most often because it is raised off the

ground.ground.

Gravitational Potential Gravitational Potential EnergyEnergy

Depends on mass and heightDepends on mass and height PE = m x g x hPE = m x g x h m is mass in kilogramsm is mass in kilograms g is acceleration caused by gravityg is acceleration caused by gravity h is distance it can fall in meters.h is distance it can fall in meters. Remember mg is weight in N so mgh is Remember mg is weight in N so mgh is

force times distance.force times distance.

Calculating PECalculating PE

A 100 kg boulder is on the edge of the cliff A 100 kg boulder is on the edge of the cliff 10 m off the ground. How much energy 10 m off the ground. How much energy does it have?does it have?

A 0.5 kg ball is thrown 15 m into the air A 0.5 kg ball is thrown 15 m into the air How much potential energy does it have at How much potential energy does it have at its highest point?its highest point?

Kinetic EnergyKinetic Energy

The energy of motionThe energy of motion Depends on two thingsDepends on two things Mass and velocityMass and velocity Twice the mass, twice the kinetic energyTwice the mass, twice the kinetic energy Twice the velocity four times the kinetic Twice the velocity four times the kinetic

energyenergy KE = KE = 1 1 mvmv22

2 2

Calculating Kinetic Calculating Kinetic EnergyEnergy

KE = KE = 1 1 mvmv22

2 2 What is the kinetic energy of a 100 kg man What is the kinetic energy of a 100 kg man

moving 5 m/s?moving 5 m/s? What is the kinetic energy of 0.5 kg ball What is the kinetic energy of 0.5 kg ball

moving at 30 m/s?moving at 30 m/s?

Mechanical EnergyMechanical Energy

The sum of the potential and kinetic The sum of the potential and kinetic energy.energy.

Before an apple falls it has all potential Before an apple falls it has all potential energyenergy

Just before it hits the ground it has all Just before it hits the ground it has all kinetic energykinetic energy

In between it has some potential energy, In between it has some potential energy, and some kinetic energyand some kinetic energy

Other forms of energyOther forms of energy

Chemical energyChemical energy – stored in the bonds – stored in the bonds between atomsbetween atoms

Reactions release or absorb energyReactions release or absorb energy TemperatureTemperature – measures the kinetic – measures the kinetic

energy of the particlesenergy of the particles HeatHeat – the total kinetic energy of the – the total kinetic energy of the

particles of a substanceparticles of a substance

Other forms of energyOther forms of energy

Nuclear energyNuclear energy- energy from changing the - energy from changing the nucleus of atomsnucleus of atoms

The sun’s energy comes from fusion – The sun’s energy comes from fusion – putting two hydrogen atoms to make putting two hydrogen atoms to make helium atomshelium atoms

E = mcE = mc22 mass is converted to energy mass is converted to energy Electricity-Electricity- the energy of charged particles the energy of charged particles LightLight- energy that can travel through - energy that can travel through

empty space in electromagnetic wavesempty space in electromagnetic waves

Conservation of energyConservation of energy

Energy can’t be created or destroyedEnergy can’t be created or destroyed The total energy remains constantThe total energy remains constant It just changes formIt just changes form

A Pendulum

PE

No KE

All KE PE

No KE

Energy is transformedEnergy is transformed

Potential to kineticPotential to kinetic But the pendulum will stop eventually.But the pendulum will stop eventually. Where does the energy goWhere does the energy go Into moving the airInto moving the air Some energy is always changed into a Some energy is always changed into a

form you don’t wantform you don’t want Friction turns motion to heat.Friction turns motion to heat. Electric cords get hotElectric cords get hot

Energy is ConservedEnergy is Conserved

All the energy can be accounted forAll the energy can be accounted for It can be hardIt can be hard Two types of systemsTwo types of systems Closed system does not let energy in or Closed system does not let energy in or

outout Used by scientists to limit variablesUsed by scientists to limit variables Open system does let energy in or outOpen system does let energy in or out Much more commonMuch more common

EfficiencyEfficiency

Not all the work done is useful workNot all the work done is useful work Some gets turned into other formsSome gets turned into other forms Often heat Often heat Efficiency = Efficiency = Useful workUseful work

Work inputWork input Or % Efficiency = Or % Efficiency = Useful workUseful work x 100% x 100%

Work inputWork input Always less than 100% efficientAlways less than 100% efficient

Perpetual Motion Perpetual Motion MachinesMachines

Machines that would run forever without Machines that would run forever without energy inputenergy input

Or machines that put out more energy Or machines that put out more energy than you put in.than you put in.

They don’t exist.They don’t exist. Would require a complete absence of Would require a complete absence of

friction.friction. Or they would break the law of Or they would break the law of

conservation of energyconservation of energy