Chapter 8Chapter 8

Work and EnergyWork and Energy

8.1 Work, Power, and Machines8.1 Work, Power, and MachinesWorkWork

What is WorkWhat is Work

Work is done only when force cause a change Work is done only when force cause a change in the motion of an object.in the motion of an object.

Work is calculated by multiplying an applied Work is calculated by multiplying an applied force by the distance over which the force is force by the distance over which the force is applied.applied.

Work = force x distanceW = F x d

8.1 Work, Power, and Machines8.1 Work, Power, and MachinesWorkWork

In the case of trying to lift a heavy object, you In the case of trying to lift a heavy object, you might apply a large force, but if the distance might apply a large force, but if the distance that the object moves is zero, there is no work that the object moves is zero, there is no work done. done.

An object has to move at least a small amount An object has to move at least a small amount for work to be done, because work is the for work to be done, because work is the product of force times distance.product of force times distance.

Work = force x distanceW = F x d

Work is measured in Joules, J.Work is measured in Joules, J.

Because work is the product of force times Because work is the product of force times distance, it is measured in units of Newtons distance, it is measured in units of Newtons

times meters or N times meters or N .. m m

1 N N .. m = 1 J = 1 kg m = 1 J = 1 kg..mm22/s/s22

8.1 Work, Power, and Machines8.1 Work, Power, and MachinesWorkWork

What is PowerWhat is Power

Power is defined as the rate at which work is Power is defined as the rate at which work is done.done.

Power = work / timeP = W / t

8.1 Work, Power, and Machines8.1 Work, Power, and MachinesPowerPower

What is PowerWhat is Power

Power is defined as the rate at which work is Power is defined as the rate at which work is done.done.

Because power is the rate at which work is Because power is the rate at which work is done, it is measured in units of Joules per done, it is measured in units of Joules per second or Watts.second or Watts.

1 N N .. m /s = 1 J / s = 1 watt m /s = 1 J / s = 1 watt

8.1 Work, Power, and Machines8.1 Work, Power, and MachinesPowerPower

Machines multiply and redirect forcesMachines multiply and redirect forces Work input equals work outputWork input equals work output Mechanical advantage tells how much a Mechanical advantage tells how much a

machine multiplies force or increases distancemachine multiplies force or increases distance– Defined as the ratio of the output force over the Defined as the ratio of the output force over the

input force or the input distance to the output input force or the input distance to the output distance.distance.

Mechanical advantage = output force / input forceOr = input distance / output distance

8.1 Work, Power, and Machines8.1 Work, Power, and MachinesMachines & Mechanical AdvantageMachines & Mechanical Advantage

The six (6) Simple MachinesThe six (6) Simple MachinesThe lever familyThe lever family

simple leversimple leverpulleypulleywheel & axlewheel & axle

The inclined plane familyThe inclined plane familysimple inclined planesimple inclined planewedgewedgescrewscrew

8.2 Simple Machines8.2 Simple Machines

First class leverFirst class leverAll first class levers have a fulcrum in the All first class levers have a fulcrum in the middle of an arm; the input force acts on middle of an arm; the input force acts on one end, and the other end supplies an one end, and the other end supplies an output force. [Claw hammer] MA > 1output force. [Claw hammer] MA > 1

8.2 Simple Machines8.2 Simple MachinesThe Three Classes of LeversThe Three Classes of Levers

Input forceOutput

force

Second class leverSecond class leverAll second class levers have a fulcrum at All second class levers have a fulcrum at one end of an arm; the input force is applied one end of an arm; the input force is applied to the other end. [wheel barrow] MA > 1to the other end. [wheel barrow] MA > 1

8.2 Simple Machines8.2 Simple MachinesThe Three Classes of LeversThe Three Classes of Levers

Input force

Output force

Third class leverThird class leverAll third class levers also have a fulcrum at All third class levers also have a fulcrum at one end of an arm; however, these levers one end of an arm; however, these levers multiply distance rather than force. multiply distance rather than force. [forearm] MA < 1[forearm] MA < 1

8.2 Simple Machines8.2 Simple MachinesThe Three Classes of LeversThe Three Classes of Levers

Input force

Output force

The mechanical advantage of pulleysThe mechanical advantage of pulleys

8.2 Simple Machines8.2 Simple MachinesPulleys are modified leversPulleys are modified levers

Input force = 150 N

Output force = 150 N

MA = 1Fixed pulley

The mechanical advantage of pulleysThe mechanical advantage of pulleys

8.2 Simple Machines8.2 Simple Machines Pulleys are modified leversPulleys are modified levers

Input force = 75 N

Output force = 150 N

MA = 2 Moving pulley

The mechanical advantage of pulleysThe mechanical advantage of pulleys

8.2 Simple Machines8.2 Simple MachinesPulleys are modified leversPulleys are modified levers

Input force = 75 N

Output force = 150 N

MA = 3 Multiple pulleys

8.2 Simple Machines8.2 Simple MachinesA wheel and axle is a lever or pulley A wheel and axle is a lever or pulley

connected to a shaftconnected to a shaft

Output force

fulcrum

8.2 Simple Machines8.2 Simple MachinesThe Inclined Plane FamilyThe Inclined Plane Family

Inclined Planes multiply and redirect Inclined Planes multiply and redirect forceforce

Input force

Output force

An inclined plane changes both the magnitude and direction of force

8.2 Simple Machines8.2 Simple MachinesThe Inclined Plane FamilyThe Inclined Plane Family

A wedge is a modified inclined planeA wedge is a modified inclined plane

8.2 Simple Machines8.2 Simple MachinesThe Inclined Plane FamilyThe Inclined Plane Family

A screw is an inclined plane wrapped A screw is an inclined plane wrapped around a cylinder.around a cylinder.

8.2 Compound Machines8.2 Compound Machines Many devices that you use every day are Many devices that you use every day are

made of more than one simple machine.made of more than one simple machine.

A machine that combines two or more A machine that combines two or more simple machines is called a compound simple machines is called a compound machine.machine.

8.3 Energy and Work8.3 Energy and Work

Energy is measured in joulesEnergy is measured in joules

While work is done only when an While work is done only when an object experiences a change in its object experiences a change in its motion, energy can be present in an motion, energy can be present in an object or a system when nothing is object or a system when nothing is happening at all.happening at all.

There are several types and forms of There are several types and forms of energy: potential, kinetic, chemical, energy: potential, kinetic, chemical, nuclear, electromagnetic, electrical, nuclear, electromagnetic, electrical, etc.etc.

8.3 Energy and Work8.3 Energy and Work

Potential EnergyPotential Energy

Potential energy is stored energy.Potential energy is stored energy.

Gravitational potential energy Gravitational potential energy depends on both mass and height:depends on both mass and height:

Gravitational PE = mass x free-fall acceleration x Gravitational PE = mass x free-fall acceleration x height.height.

PE = m g hPE = m g h

Do practice problems on page 265Do practice problems on page 265

8.3 Energy and Work8.3 Energy and Work

Kinetic EnergyKinetic Energy

The energy that a n object has The energy that a n object has because it is in motion is called because it is in motion is called kinetic energy.kinetic energy.

Kinetic energy depends on mass and Kinetic energy depends on mass and velocity.velocity.

KE = ½ x mass x velocity squaredKE = ½ x mass x velocity squared

KE = ½ m vKE = ½ m v22

8.4 Conservation of Energy 8.4 Conservation of Energy

Potential energy can become kinetic Potential energy can become kinetic energyenergy

Kinetic energy can become potential Kinetic energy can become potential energyenergy

Energy can neither be created or Energy can neither be created or destroyed!destroyed!