Work, Power and Work, Power and EnergyEnergy

Chapters 4 & 5Chapters 4 & 5

What is Energy?What is Energy?

Energy is the ability to cause changeEnergy is the ability to cause change Energy has different forms—Energy has different forms—

electrical, chemical, radiant, and electrical, chemical, radiant, and thermalthermal

Kinetic EnergyKinetic Energy

Energy of a moving object due to its Energy of a moving object due to its motion; measured in Joules (J)= kg∙mmotion; measured in Joules (J)= kg∙m22/s/s22

KE = ½ mvKE = ½ mv22

A baseball with a mass of 0.15 kg is A baseball with a mass of 0.15 kg is moving at a speed of 40 m/s. What is the moving at a speed of 40 m/s. What is the baseball’s kinetic energy?baseball’s kinetic energy?

A sprinter has a mass of 80.0 kg and a A sprinter has a mass of 80.0 kg and a kinetic energy of 4000 J. What is the kinetic energy of 4000 J. What is the sprinter’s speed?sprinter’s speed?

Potential EnergyPotential Energy

Stored energy due to positionStored energy due to position Elastic potential energy—stored Elastic potential energy—stored

energy that can stretch or compressenergy that can stretch or compress Chemical potential energy—energy Chemical potential energy—energy

stored in chemical bondsstored in chemical bonds

Gravitational PEGravitational PE Stored energy due to position above Stored energy due to position above

Earth’s surfaceEarth’s surface GPE= mgh (in Joules); g = 9.8 m/sGPE= mgh (in Joules); g = 9.8 m/s22

Find the GPE of a coffee mug with a mass Find the GPE of a coffee mug with a mass of 0.3 kg that is on a counter top 1 m high of 0.3 kg that is on a counter top 1 m high above the ground.above the ground.

How high above the ground is a baseball How high above the ground is a baseball with a mass of 0.15 kg that has a GPE of with a mass of 0.15 kg that has a GPE of 73.5 J?73.5 J?

A rock climber is 200 m above the ground A rock climber is 200 m above the ground and has a GPE of 117,600 J. What is the and has a GPE of 117,600 J. What is the rock climber’s mass?rock climber’s mass?

Changing GPEChanging GPE

As height increases, GPE increasesAs height increases, GPE increases As object falls, it loses GPE and gains As object falls, it loses GPE and gains

KE; when it hits the ground, GPE is KE; when it hits the ground, GPE is zero and all GPE has transformed zero and all GPE has transformed into KEinto KE

As ball leaves bat, it has mostly KE. As it rises, velocity decreases and GPE increases. At highest point, ball has its maximum GPE and only KE is due to forward motion. As ball falls, GPE decreases while KE increases as ball moves faster.

Changing Forms of EnergyChanging Forms of EnergyLightbulb converts electrical energy into light and thermal energy. What are some other devices that convert electricity into thermal energy?

In a car, spark plug fires initiating the conversion of chemical potential energy into thermal energy; as hot gases expand, thermal energy is converted into kinetic energy.

Mechanical EnergyMechanical Energy

Is the total amount of potential and Is the total amount of potential and kinetic energy in a systemkinetic energy in a system

Energy in a swing—look at pg. 110Energy in a swing—look at pg. 110

Law of Conservation of EnergyLaw of Conservation of Energy

Energy cannot be created or Energy cannot be created or destroyeddestroyed

Effect of FrictionEffect of Friction

Mechanical energy is converted into Mechanical energy is converted into thermal energy (air resistance, thermal energy (air resistance, friction of hooks, etc.)friction of hooks, etc.)

Appears that energy is lost (as swing slows down), but energy is actually transformed.

Converting Mass into EnergyConverting Mass into EnergyA small amount of mass converted into tremendous amount of energy.

In both processes, energy is conserved if energy content of masses involved are included.

Energy Conversions in the BodyEnergy Conversions in the Body

Complex chemical and physical Complex chemical and physical processes processes

Food energy (1 Calorie—4184 Joules)Food energy (1 Calorie—4184 Joules) Fat about 9 C per gram; proteins and Fat about 9 C per gram; proteins and

carbs about 4 C of energy per gramcarbs about 4 C of energy per gram

Work Work Work makes something moveWork makes something move Applied force must make something Applied force must make something

move; movement must be in same move; movement must be in same direction as forcedirection as force

Work is only done when force is directly applied to the object and when force is in same direction of distance.

Work and EnergyWork and Energy When work is done, energy is always When work is done, energy is always

transferred; when you do work on object, transferred; when you do work on object, you increases its energyyou increases its energy

Work (J) = F (N) x d (m)Work (J) = F (N) x d (m) A couch is pushed with a force of 75 N and A couch is pushed with a force of 75 N and

moves a distance of 5 m across the floor. moves a distance of 5 m across the floor. How much work is done in moving the How much work is done in moving the couch?couch?

The brakes on a car do 240,000 J of work The brakes on a car do 240,000 J of work in stopping the car. If the car travels a in stopping the car. If the car travels a distance of 50 m while the brakes are distance of 50 m while the brakes are being applied, what is the force the brakes being applied, what is the force the brakes exert on the car?exert on the car?

PowerPower

Rate at which work is done; amount of Rate at which work is done; amount of work done per secondwork done per second

Power (in watts)= work (J)/time (s)Power (in watts)= work (J)/time (s) P=W/tP=W/t If lifting a babe from a crib, 50J of work are If lifting a babe from a crib, 50J of work are

done. How much power is needed if the done. How much power is needed if the baby is lifted in 2.0 s?baby is lifted in 2.0 s?

The power produced by an electric motor The power produced by an electric motor is 500 W. How long will it take the motor is 500 W. How long will it take the motor to do 10,000 J of work?to do 10,000 J of work?

Power and EnergyPower and Energy

Power is also the rate at which Power is also the rate at which energy is transferredenergy is transferred

P= E/tP= E/t

What is a Machine?What is a Machine?

A device that makes doing work A device that makes doing work easier.easier.

Make work easier by increasing the Make work easier by increasing the force that can be applied to an objectforce that can be applied to an object

Whether the mover slides the chair up the ramp or lifts it directly into the truck, she will do the same amount of work. Doing the work over a longer distance allows her to use less force.

Changing DirectionChanging Direction

Some machines change the direction Some machines change the direction of the applied forceof the applied force

Input and Output ForcesInput and Output Forces

FFinin = force applied to the machine = force applied to the machine FFoutout = force applied by the machine = force applied by the machine W W inin= work done by you on the machine= work done by you on the machine W W outout= work done by the machine= work done by the machine W W in = in = W W out out or F or Fin in ddinin = F = Foutoutddoutout

Remember WRemember Woutout is always smaller the W is always smaller the Winin

Ideal machine, WIdeal machine, Winin = W = Woutout

Mechanical AdvantageMechanical Advantage

Ratio of output force to input a\forceRatio of output force to input a\force MA = FMA = Foutout/F/Finin

Ideal MA (IMA) is without friction and Ideal MA (IMA) is without friction and calculated by dcalculated by dinin/d/doutout

Calculating EfficiencyCalculating Efficiency

Efficiency of a machineEfficiency of a machine Efficiency (%) = WEfficiency (%) = Woutout/W/Winin x 100% x 100% Efficiency may be increased by Efficiency may be increased by

reducing friction (lubricant)reducing friction (lubricant)

Simple MachinesSimple Machines

Machine that does work with only Machine that does work with only one movement of the machineone movement of the machine

Lever, pulley, wheel and axle, Lever, pulley, wheel and axle, inclined plane, screw, and wedgeinclined plane, screw, and wedge

Screw and wedge are modified Screw and wedge are modified inclined planesinclined planes

LeversLevers

First class—output force is always in the opposite direction to the input force; fulcrum in between the forces

Second class—output force located between input and fulcrum; wheelbarrow; output force greater than input force

Third class—input force is applied between the output force and the fulcrum; baseball bat; output force is always less than input force and distance over which the output force is applied is increased. IMA = Lin/Lout

(length input arm/length output arm)

PulleysPulleys

Axle of the pulley acts as a fulcrum in Axle of the pulley acts as a fulcrum in a fixed pulley (first class lever)a fixed pulley (first class lever)

Fixed pulley only changes the Fixed pulley only changes the direction of force, IMA = 1direction of force, IMA = 1

Movable pulley, IMA = 2

Block and Tackle (fixed and movable pulleys); IMA = 4

Wheel and AxleWheel and Axle

Simple machine with a shaft or axle Simple machine with a shaft or axle attached to the center of a larger attached to the center of a larger wheel so that the wheel and axle wheel so that the wheel and axle rotate togetherrotate together

IMA = radius of wheel (m)/radius of IMA = radius of wheel (m)/radius of axle (m) or raxle (m) or rww/r/raa

Gears—may changeGears—may change

direction of forcedirection of force

Inclined PlanesInclined Planes

A sloping surface that reduces the A sloping surface that reduces the amount of force to do workamount of force to do work

IMA= length of slope (m)/height of IMA= length of slope (m)/height of slope (m) or l/hslope (m) or l/h

Screw is inclined plane wrapped around a cylindrical post (IMA is larger if threads are closer together)

Scissors—two wedges together.

Compound MachinesCompound Machines

Two or more simple machines Two or more simple machines operating togetheroperating together

Can openers, scissors, cars, etc. Can openers, scissors, cars, etc.