chapter 3 partc

8/6/2019 Chapter 3 PartC

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Chapter 3 Energy & WorkChapter 3 Energy & Work

Sherry YinSherry Yin


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EnergyEnergy Energy is the ability to do

work.

Work is the energy

transferred by the force

You must have energy to

perform work; you don't

necessarily perform workif you have energy.

Unit: Joules (J)

Energy is the ability to do

work.

Work is the energy

transferred by the force

You must have energy to

perform work; you don't

necessarily perform workif you have energy.

Unit: Joules (J)


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Kinetic energyKinetic energy

The kinetic energy ofan object is the energywhich it possesses dueto its motion.

It is defined as the workneeded to accelerate abody of a given massfrom rest to its stated

velocity. Unit: Joules (J)

Mathematically:

The kinetic energy ofan object is the energywhich it possesses dueto its motion.

It is defined as the workneeded to accelerate abody of a given massfrom rest to its stated

velocity. Unit: Joules (J)

Mathematically:

W !

1

2mv

2


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Kinetic EnergyKinetic Energy

F

mm

W ! F S! F1

2at

2

a!F

m p ! F1

2

F

m

t2 !

1

2

F2

m

t2 !

1

2

mF

m

t

2

a !v f v i

t

vi ! 0 pF

m!v f

t v f !

F

m t

W !1

2

mvf

2

EK=


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ExerciseExercise

Determine the kinetic energy of a

600 kg roller coaster car that is

moving with a velocity of20 m/s.

1.2v 105 J

What if we double the mass?

2.4 v 105 J

What if the roller coaster wasmoving with twice the velocity?

4.8 v 105 J

Determine the kinetic energy of a

600 kg roller coaster car that is

moving with a velocity of20 m/s.

1.2v 105 J

What if we double the mass?

2.4 v 105 J

What if the roller coaster wasmoving with twice the velocity?

4.8 v 105 J


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ExerciseExercise

Missy Diwater, the former platform diver

for the Ringling Brother's Circus, had a

kinetic energy of10000 J just prior to

hitting the bucket of water. If at thatmoment, Missy's velocity is 20 m/s , then

what is her mass?

50 kg

Missy Diwater, the former platform diver

for the Ringling Brother's Circus, had a

kinetic energy of10000 J just prior to

hitting the bucket of water. If at thatmoment, Missy's velocity is 20 m/s , then

what is her mass?

50 kg


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Work-K

inetic Energy TheoremWork-K

inetic Energy Theorem The work done on an object by a net force

equals the change in kinetic energy of the

object:

Mathematically:

If the velocity of an object increases

(vf> vi) , then W > 0; vise versa.

The work done on an object by a net force

equals the change in kinetic energy of the

object:

Mathematically:

If the velocity of an object increases

(vf> vi) , then W > 0; vise versa.

F S1

2

mvf2

1

2

mv i2

Wnet ( KE KE fKEi


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Work-Energy TheoremWork-Energy Theorem

With all the info in the above, please calculate the

final velocity of the box.

With all the info in the above, please calculate the

final velocity of the box.

60 m/s


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Work-Energy TheoremWork-Energy Theorem

The ice-skater with a

mass of80 kg glides

alone a straight line

for30 m. His initialvelocity is 10 m/s and

his final velocity is 5

m/s. Determine the

friction. 100 N

The ice-skater with a

mass of80 kg glides

alone a straight line

for30 m. His initialvelocity is 10 m/s and

his final velocity is 5

m/s. Determine the

friction. 100 N


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Potential EnergyPotential Energy

Potential energy is the energy stored in a body or

in a system due to its position or configuration.

Potential energy is the energy stored in a body or

in a system due to its position or configuration.

Elasticpotentialenergy

Chemicalpotential energy

Gravitationalpotential energy

Electricalpotential energy


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Gravitational potential energyGravitational potential energy

Gravitational potential energy is the energy

stored in an object as the result of its vertical

position or height.

The energy is stored as the result of the

gravitational attraction of the Earth for the

object.

W = FS

PEgrav= Gh = mgh

Gravitational potential energy is the energy

stored in an object as the result of its vertical

position or height.

The energy is stored as the result of the

gravitational attraction of the Earth for the

object.

W = FS

PEgrav= Gh = mgh


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Gravitational potential energyGravitational potential energy P = mgh

m represents the mass of the object;

h represents the height of the object;

g represents the gravity (9.8 m/s2)

To determine the gravitational potentialenergy of an object, a zero height positionmust first be arbitrarily assigned.

Typically, the ground isconsidered to be a

position of zero height.

P = mgh

m represents the mass of the object;

h represents the height of the object;

g represents the gravity (9.8 m/s2)

To determine the gravitational potentialenergy of an object, a zero height positionmust first be arbitrarily assigned.

Typically, the ground isconsidered to be a

position of zero height.


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ExerciseExercise

The gravitationalP

E of an object is directlyproportional to its height

What is the potential energy at the other positions

shown on the stair steps and the incline

A: 40 J; B: 30 J; C: 20 J; D: 10 J; E: 0 J; F: 0 J

The gravitationalP

E of an object is directlyproportional to its height

What is the potential energy at the other positions

shown on the stair steps and the incline

A: 40 J; B: 30 J; C: 20 J; D: 10 J; E: 0 J; F: 0 J


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ExerciseExercise

A thief with a mass of80kg

is trying to escape from the

cops. He is running on the

roofs; the height of the

buildings are 50m, 52 m, 55

m. What is the gravitational

potential energy of the man

on each of the building?

39200 J

40768 J

43120 J

A thief with a mass of80kg

is trying to escape from the

cops. He is running on the

roofs; the height of the

buildings are 50m, 52 m, 55

m. What is the gravitational

potential energy of the man

on each of the building?

39200 J

40768 J

43120 J


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Conservation of EnergyConservation of Energy

The total amount of energy in an isolated systemremains constant over time

The total amount of energy in an isolated systemremains constant over time

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