EnergyEnergy

Chapter 5Chapter 5

What is energy?What is energy?

The property of an object that allows it to The property of an object that allows it to produce a change in itself or its produce a change in itself or its environment.environment.

In this class we will studyIn this class we will studyWorkWorkGravitational Potential EnergyGravitational Potential EnergyKinetic EnergyKinetic EnergySpring Potential EnergySpring Potential Energy

WorkWork

Generic term for energy given to an object from Generic term for energy given to an object from an outside source or energy lost to the outside an outside source or energy lost to the outside from an object.from an object.

W=FdcosW=Fdcosθθ θθ is the angle between the force and the is the angle between the force and the

displacement.displacement. Unless explicitly told otherwise, assume that the Unless explicitly told otherwise, assume that the

angle between force and displacement is 0.angle between force and displacement is 0. When When θθ=90=90oo, cos, cosθθ=0 so no work is done.=0 so no work is done.

WorkWork

Unit for energy is the Joule.Unit for energy is the Joule. Other units: calories, kilowatt hours, Other units: calories, kilowatt hours, Negative work means that the object in question Negative work means that the object in question

is losing energy i.e. friction.is losing energy i.e. friction. Sometimes labeled Q when referring to heat Sometimes labeled Q when referring to heat

specifically.specifically.

2

2

2)(

s

kgmm

s

kgmNmJ

Practice ProblemsPractice Problems

A man lifts a 150kg barbell up 2.5m. How A man lifts a 150kg barbell up 2.5m. How much work does he do?much work does he do?

A woman pushes a penguin up an incline A woman pushes a penguin up an incline of angle 32 degrees. If she pushes of angle 32 degrees. If she pushes horizontally and does 5000J of work, the horizontally and does 5000J of work, the penguin moves 10m. How much force penguin moves 10m. How much force does she exert?does she exert?

PowerPower

The rate at which energy is transferred.The rate at which energy is transferred.

t

Energy in Change

t

WP

Unit is a WattUnit is a Watt 1W=1J/s, so 1J=1Ws and we can convert from 1W=1J/s, so 1J=1Ws and we can convert from

Joules to kWhr.Joules to kWhr. English unit is horsepower.English unit is horsepower.

Power ProblemPower Problem You leave the 65W bulb in your room on for two weeks. How much energy does it use?You leave the 65W bulb in your room on for two weeks. How much energy does it use? If the electric company charges 12.5 cents per kilowatt hour, how much does it cost your parents?If the electric company charges 12.5 cents per kilowatt hour, how much does it cost your parents?

Mechanical EnergyMechanical Energy

Energy due to movement or position.Energy due to movement or position.Kinetic energy-energy of motionKinetic energy-energy of motionPotential energy-energy due to position or Potential energy-energy due to position or

shape of an object.shape of an object.Total energy-the sum of all potential and Total energy-the sum of all potential and

kinetic energies of an object or system.kinetic energies of an object or system.

Kinetic EnergyKinetic Energy

2

21 mvKE

Depends on velocity and massDepends on velocity and mass Doubling velocity quadruples the energyDoubling velocity quadruples the energy If a bird flying at 5m/s has a kinetic energy of If a bird flying at 5m/s has a kinetic energy of

1.4J, what is the mass of the bird?1.4J, what is the mass of the bird? What is the difference in kinetic energy between What is the difference in kinetic energy between

a 2000kg car moving at 5mph,10mph, and a 2000kg car moving at 5mph,10mph, and 15mph.15mph.

Work Kinetic Energy TheoremWork Kinetic Energy Theorem

KEW A car accelerates on a frictionless road at a rate A car accelerates on a frictionless road at a rate

of 3.6m/sof 3.6m/s22. If the car has a final velocity of . If the car has a final velocity of 42m/s and accelerates over 192m, what was its 42m/s and accelerates over 192m, what was its initial velocity?initial velocity?

Gravitational Potential EnergyGravitational Potential Energy

mghPE H is subjectiveH is subjective Only really measures changes in PEOnly really measures changes in PE Can be negative whereas KE cannotCan be negative whereas KE cannot What is the potential energy of a roller coaster car of What is the potential energy of a roller coaster car of

mass 450kg that is 17m off of the ground?mass 450kg that is 17m off of the ground? A 45kg diver dives off of a board which is 7m above the A 45kg diver dives off of a board which is 7m above the

bottom of the pool. If the pool is 2m deep, what is the bottom of the pool. If the pool is 2m deep, what is the change in potential energy of the diver from when she is change in potential energy of the diver from when she is on the board to when she hits the surface of the water? on the board to when she hits the surface of the water?

Conservation of EnergyConservation of Energy

In a closed system, energy cannot be In a closed system, energy cannot be created or destroyed, only changed from created or destroyed, only changed from one form to another.one form to another.

Closed systemClosed systemCalculate total energy before and after and Calculate total energy before and after and

set them equal to each other.set them equal to each other.Energy added to a system is work on the Energy added to a system is work on the

initial side, energy lost by the system is initial side, energy lost by the system is work on the final side.work on the final side.

PracticePractice

A rock is dropped off of a 100m high cliff. A rock is dropped off of a 100m high cliff. How fast is it going right before it hits the How fast is it going right before it hits the ground?ground?

A rock is thrown off of a cliff at an angle of A rock is thrown off of a cliff at an angle of 37 degrees above the horizontal at a 37 degrees above the horizontal at a speed of 21m/s. If it is going 43m/s when speed of 21m/s. If it is going 43m/s when it hits a 2.5m tall man in the head, how it hits a 2.5m tall man in the head, how high is the cliff?high is the cliff?

FrictionFriction

Takes energy from the system and wastes it in Takes energy from the system and wastes it in heat.heat.

It exerts a force over a distance so:It exerts a force over a distance so:

dFW frfr

PracticePractice

A 6kg tuna salad sandwich starts from rest and A 6kg tuna salad sandwich starts from rest and slides down a 20 degree incline that is 4m high. slides down a 20 degree incline that is 4m high. If the final velocity of the box is 7m/s what is the If the final velocity of the box is 7m/s what is the work done by friction? What is the magnitude of work done by friction? What is the magnitude of the frictional force?the frictional force?

A 10kg wagon full of bananas, starting from rest, A 10kg wagon full of bananas, starting from rest, is pulled with horizontally with a 25N force is pulled with horizontally with a 25N force across level ground. If the coefficient of friction across level ground. If the coefficient of friction between the wagon and the ground is 0.2, how between the wagon and the ground is 0.2, how quickly is the wagon moving after 25m? quickly is the wagon moving after 25m?

Additional ProblemsAdditional Problems

If the 2kg ball above starts with an initial velocity of If the 2kg ball above starts with an initial velocity of 6.7m/s at 2m but only rolls up to a height of 3.1m, how 6.7m/s at 2m but only rolls up to a height of 3.1m, how much work is done by friction?much work is done by friction?

A driver has a truck full of chickens. The chickens and A driver has a truck full of chickens. The chickens and truck have a combined mass of 2200kg. As the truck truck have a combined mass of 2200kg. As the truck driver coasts down a frictionless hill, 300kg of the driver coasts down a frictionless hill, 300kg of the chickens fly out of the truck. If the hill is 11m high and chickens fly out of the truck. If the hill is 11m high and his initial velocity is 12m/s, how fast is he going at the his initial velocity is 12m/s, how fast is he going at the bottom of the hill?bottom of the hill?

SpringsSprings

SpringsSprings

22

1 kxPEs x=How much the spring is streched or x=How much the spring is streched or

compressed from its equilibrium position. compressed from its equilibrium position. Measured in meters.Measured in meters.

k=Spring constant, a measure of how stiff the k=Spring constant, a measure of how stiff the spring is. The stiffer the spring, the higher the k.spring is. The stiffer the spring, the higher the k.

Measured in N/mMeasured in N/m

Practice ProblemsPractice Problems

A spring attached to a 2kg block is stretched A spring attached to a 2kg block is stretched horizontally 12cm from its equilibrium position. If horizontally 12cm from its equilibrium position. If the block is traveling at 0.89m/s when the spring the block is traveling at 0.89m/s when the spring gets back to its equilibrium position, what is the gets back to its equilibrium position, what is the spring constant of the spring?spring constant of the spring?

A bungee jumper jumps off of a bridge at a A bungee jumper jumps off of a bridge at a speed of 2m/s. The bungee cord has a spring speed of 2m/s. The bungee cord has a spring constant of 32N/m and is 12m long when constant of 32N/m and is 12m long when unstreched. If the jumper falls a distance of 45m unstreched. If the jumper falls a distance of 45m before switching directions, what is her mass?before switching directions, what is her mass?