Lect08

04/13/23 Physics 103, Spring 2009, U. Wisconsin

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Physics 103: Physics 103: Lecture 8Lecture 8Work and EnergyWork and Energy

Work and Kinetic Energy Reminders:

Midterm Exam I, Tuesday February 24, 5:45 PMMaterial from Chapters 1-4 inclusiveOne page of notes (8.5” x 11”) allowed20 multiple choice questions, 90 minutesScantron will be used - bring #2 HB pencils + calculator


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WorkWork

Requires non-zero external force (F) and displacement (x) in the direction () of the force

Units are Newton-meter defined as a joule also 1 calorie (energy raises a gm of water 1 degree C) 1 cal = 4.186 J

W = F x cos

+ work is done on the object by the external force, if there is a displacement in the same direction

- work is done on an object by an external force, if there is a displacement in the opposite direction.


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Work and Force DirectionWork and Force Direction

Work is done in lifting the box


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A woman holds up a bowling ball in a fixed position. The work she does on the ball

1. Depends on the weight of the ball.

2. Cannot be calculated without more information.

3. Is equal to zero.

Although the woman is exerting force against gravity to hold the bowling ball up, she has not shifted its position. Therefore, the work done by her on the ball is zero.

Question 1Question 1


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A man pushes a very heavy load across a horizontal floor. The work done by gravity on the load

1. Depends on the weight of the load.

2. Depends on the coefficient of kinetic friction between the load and the floor.

3. Is equal to zero.

The load is moving horizontally, where as gravitational force is vertical.



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Lecture 9, Lecture 9, Preflight 1 & 2Preflight 1 & 2

You are towing a car up a hill with constant velocity. The work done on the car by the normal force is:

1. positive2. negative3. zero

W

T

FN V

Normal force is perpendicular to displacement

cos = 0

correct

Pretty SureNot Quite SureJust Guessing


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You are towing a car up a hill with constant velocity. The work done on the car by the gravitational force is:


There is a non-zero component of gravitational force pointing opposite the direction of motion.

W

T

FN V

correct

0

10

20

30

40

50

60

70

Preflight 8.1

ABC



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You are towing a car up a hill with constant velocity. The work done on the car by the tension force is:


T is pointing in the direction of motion - therefore, work done by this force is positive.

W

T

FN V

correct

0

10

20

30

40

50

60

70

80

90

Preflight 8.5

ABC



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You are towing a car up a hill with constant velocity. The total work done on the car by all forces is:


Constant velocity implies that there is no net force acting on the car, so there is no work being done overall

W

T

FN V

correct



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EnergyEnergy

Energy is that quality of a substance or object which causes something to happen. Perhaps, one could term it the capability of exerting forces.

The vagueness of the definition is due to the fact that energy can result in many effects.

It is convertible into other forms without loss (i.e it is conserved)

Kinetic EnergyElectrical EnergySolar EnergyChemical EnergyNuclear EnergyGravitational Energy ……….


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Converting Form of EnergyConverting Form of Energy


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Kinetic Energy Kinetic Energy

The “energy of motion”.Work done on the object increases its

energy, -- by how much? (i.e. how to calculate the value?)

F = maW = F d

W = (ma) d

V = V + 2 a d2 2

0

W = mV - m VW = mV - m V12

12

22

0

a = 2 d

V - V22

0

2 d

V - V22

0W = m d

KE = mVKE = mV12

2

Work-kinetic energy theorem


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When you do positive work on an object, its kinetic energy

1. increases.

2. decreases.

3. remains the same.

4. need more information about the way the work was done

Work-energy theorem:

€

net W =12mv2 −1

2mv02



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Work Done by GravityWork Done by Gravity Change in gravitational potential

energy,

PEg = mgh

true for any path : h, is simply the height difference, yfinal - yinitial

A falling object converts gravitational potential energy to its kinetic energy

Work needs to be done on an object to move it vertically up - work done is the same no matter what path is taken


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Suppose you want to ride your mountain bike up a steep hill. Two paths lead from the base to the top, one twice as long as the other. Compared to the average force exerted along the short path, Fav, the average force you exert along the longer path is

1. undetermined, because it depends on the time taken

2. Fav / 2

3. Fav

4. 2 Fav

correct

Gravitational potential energy gained is the same for both cases

It is equal to average force exerted times distance

Since distance traveled is twice, the Fav is one-half



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Two marbles, one twice as heavy as the other, are dropped to the ground from the roof of a building. Just before hitting the ground, the heavier marble has

1. as much kinetic energy as the lighter one

2. twice as much kinetic energy as the lighter one

3. half as much kinetic energy as the lighter one

4. no kinetic energy

correct

Final velocity of the two marbles is the same

Kinetic energy is proportional to mass



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Hooke’s LawHooke’s Law

Force exerted to compress a spring is proportional to the amount of compression.

€

Fs = −kx

€

PE s =1

2kx 2


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The potential energy of a spring is

1. proportional to the amount the spring is stretched.

2. proportional to the square of the amount the spring is

stretched.

3. proportional to the amount the spring is compressed.

Question 6Question 6 Stretching


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Conservation of EnergyConservation of Energy

Lect08

Education

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