work and energy
TRANSCRIPT
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Work and EnergyChapter 9Pg.144-
+
Work
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What do you think?List five examples of things you have done
in the last year that you would consider work.
Based on these examples, how do you define work?
+Work
In physics, work is the magnitude of the force (F) times the magnitude of the displacement (d) in the same direction as the force.
W = Fd
What are the SI units for work? Force units (N) distance units (m) N•m are also called joules (J).
How much work is 1 joule? Lift an apple weighing about 1 N from the floor
to the desk, a distance of about 1 m.
+Work
If we lift two loads, we do twice as much work as lifting one load the same distance, because the force needed is twice as great.
If we lift one load twice as far, we do twice as much work because the distance is twice as great.
+Work
Work is done in lifting the barbell. If the barbell could be lifted twice as high, the weight lifter would have to do twice as much work.
+Work
While the weight lifter is holding a barbell over his head, he may get really tired, but he does no work on the barbell.
Work may be done on the muscles by stretching and squeezing them, but this work is not done on the barbell.
When the weight lifter raises the barbell, he is doing work on it.
+Classroom Practice Problem
A 20.0 kg suitcase is raised 3.0 m above a platform. How much work is done on the suitcase?Answer: 600 J
Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package?W = Fd = 60 N × 4 m = 240 J
+Work is a Scalar
Work can be positive or negative but does not have a direction.
+Sign of Work is Important
Work is positiveForce is in the same direction as the
displacement
Work is negativeForce is in a different direction as the
displacement
Sing of the net work lets you know if the object is speeding up or down+ for speeding up and work is being on object- for slowing down and work is done by object
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Energy
+Kinetic Energy
Energy associated with an object in motion
Since
Then
Finally
2 2
( )2
f inet
v vW m
2 21 1
2 2net f iW mv mv
Wnet = Fd = madv2
f = v2i + 2ad2 2
2f iv v
ad
2 2
( )2
f inet
v vW m
2 21 1
2 2net f iW mv mv
+Kinetic EnergyKinetic energy depends on speed and mass
The net work done on a body equals its change in kinetic energy
SI units for KEkg•m2/s2 or N•m or Joule (J)
+Example
A 7.0 Kg bowling ball moves at 3.0 m/s. How fast must a 2.45g ping pong ball move in order to have the same kinetic energy as the bowling ball? Is the speed reasonable for the ping pong ball?
Given:
Bowling ball: m- 7.0 kg v= 3.0m/s
Ping pong: m= 2.45 g (this= 0.00245kg)v-??
+Example
KE= ½ mv2
KE= ½ (7)(32)
KE= 31.5 J
Rearrange Equation to get v by itself
v = 160.36 m/s
2(31.5)
0.00245v
2KEv
m
+Classroom Practice Problems
A 6.00 kg cat runs after a mouse at 10.0 m/s. What is the cat’s kinetic energy?Answer: 3.00 x 102 J or 300 J
Suppose the above cat accelerated to a speed of 12.0 m/s while chasing the mouse. How much work was done on the cat to produce this change in speed?Answer: 1.32 x 102 J or 132 J
+Work and Kinetic Energy
KE is the work an object can do if the speed changes.
Wnet is positive if the speed increases, and negative is speeds decrease
You must include all the forces that do work on the object in calculating the net work done
+Potential Energy
Energy associated with an object’s potential to move due to an interaction with its environment basically its stored energyA book held above the deskAn arrow ready to be released from the bow
Some types of PE are listed below.GravitationalElasticElectromagnetic
+Gravitational Potential Energy
Energy associated with an object due to the object’s position relative to a gravitational source
SI unit is still a Joule
The height (h) depends on the “zero level” chosen where PEg= 0.
+Elastic Potential Energy
The energy available for use in deformed elastic objects Rubber bands, springs in trampolines, pole-vault
poles, muscles
For springs, the distance compressed or stretched = x
+Elastic Potential Energy
The spring constant (k) depends on the stiffness of the spring.Stiffer springs have higher k values.Measured in N/m
Force in newtons needed to stretch a spring 1.0 meters
+Example
A 70.0kg stuntman is attached to a bungee cord with an unstretched length of 15m. He jumps off a bridge from a height of 50m. When he finally stops the cord has a stretched length of 44m. Assuming the spring constant is 71.8 N/m, what is the total PE relative to the water when the man stops falling?
+Example
Given:
m= 70kg k= 71.8 N/mg= 10m/s2
h= 50m – 44m= 6m
x= 44m – 15m= 29m
PEg= mgh
PEelastic = ½ k x2
PEtotal= PEg + PEelastic
+Example
PEg= mgh
PEg= (70)(10)(6)
PEg= 4200 J
PEtotal= PEg + PEelastic
PEtotal= 4200 + 30191.9
PEtotal= 34391.9J
PEelastic = ½ k x2
PEelastic= ½ (71.8)(292)
PEelastic= 30191.9J
+Classroom Practice Problems
When a 2.00 kg mass is attached to a vertical spring, the spring is stretched 10.0 cm such that the mass is 50.0 cm above the table.What is the gravitational potential energy
associated with the mass relative to the table?Answer: 9.81 J
What is the spring’s elastic potential energy if the spring constant is 400.0 N/m?Answer: 2.00 J
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5.3 Conservation of EnergyPg. 173-178
+Mechanical Energy (ME)
ME = KE + PEg + PEelastic
Does not include the many other types of energy, such as thermal energy, chemical potential energy, and others
ME is not a new form of energy.Just a combination of KE and PE
+Conservation of Mechanical EnergyThe sum of KE and PE remains
constant.
One type of energy changes into another type. For the falling book, the PE of the book
changed into KE as it fell. As a ball rolls up a hill, KE is changed into
PE.
+Example
Starting from rest, a child zooms down a frictionless slide from an initial height of 3.0m. What is her speed at the bottom of the slide? Her mass is 25kg.
Given:
vi= 0m/s hi= 3m m=25kg
vf= ?? hf=0m
+Example
*Choose your equations
PE= mgh
PEf= (25)(10)(0)
PEf= 0J
PEi= (25)(10)(3)
PEi= 750J
KE= ½ mv2
KEf= ½ (25)v2
KEf= ??
KEi= ½ (25)(02)
KEi= 0J
+Example
*Put together
PEi+ KEi= PEf+ KEf
750 + 0 = 0 + ½ (25)vf2
750= 12.5 vf2
vf2 = √60
vf= 7.75m/s
+Classroom Practice Problems
Suppose a 1.00 kg book is dropped from a height of 2.00 m. Assume no air resistance.Calculate the PE and the KE at the instant the book is
released.Answer: PE = 19.6 J, KE = 0 J
Calculate the KE and PE when the book has fallen 1.0 m. (Hint: you will need an equation from Chapter 2.)Answer: PE = 9.81 J, KE = 9.81 J
Calculate the PE and the KE just as the book reaches the floor.Answer: PE = 0 J, KE = 19.6 J
+Table of Values for the Falling Book
h (m) PE(J) KE(J) ME(J)
0 19.6 0 19.6
0.5 14.7 4.9 19.6
1.0 9.8 9.8 19.6
1.5 4.9 14.7 19.6
2.0 0 19.6 19.6
+Conservation of Energy Acceleration does not have to be constant.
ME is not conserved if friction is present. If friction is negligible, conservation of ME is
reasonably accurate. A pendulum as it swings back and forth a few
times
Consider a child going down a slide with friction. What happens to the ME as he slides down?
Answer: It is not conserved but, instead, becomes less and less.
The “lost” energy? is converted into nonmechanical energy (thermal energy).
+Classroom Practice Problems
A small 10.0 g ball is held to a slingshot that is stretched 6.0 cm. The spring constant is 2.0 102 N/m.What is the elastic potential energy of the
slingshot before release?What is the kinetic energy of the ball right
after the slingshot is released?What is the ball’s speed at the instant it
leaves the slingshot?How high does the ball rise if it is shot
directly upward?
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Power
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What do you think?
Two cars are identical with one exception. One of the cars has a more powerful engine. How does having more power make the car behave differently? What does power mean?What units are used to measure power?
+Power
The rate at which work is done or energy is transferredEnergy used or work done per second
If we substitute W for Fd then FdP
t
+Power
The unit of power is the joule per second, also known as the watt.
One watt (W) of power is expended when one joule of work is done in one second.
One kilowatt (kW) equals 1000 watts.
One megawatt (MW) equals one million watts.
+Power
SI units for power are J/s. Called watts (W) Equivalent to kg•m2/s3
Horsepower (hp) is a unit used in the Avoirdupois system. 1.00 hp = 746 W
+Watts
These bulbs all consume different amounts of power.
A 100 watt bulb consumes 100 joules of energy every second.
+Example
A 193kg curtain need to be raised 7.5m, at a constant speed, in as close to 5 sec as possible. Unsure which motor would be the best 3 motors were bought. Power ratings are 1.0kW, 3.5kW, and 5.5kW. Which motor is best for the job?
Given:
m= 193kg d= 7.5m t= 5 sec P=??
+Example
P=2895 W or 2.895kW
So the best motor would be the 3.5kW motor
+Classroom Practice Problems
Two horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.Calculate the power delivered by the
horses.How much work is done by the two
horses?
Answers: 1.0 x 103 W and 6.0 x 105 J
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Now what do you think?
Two cars are identical with one exception. One of the cars has a more powerful engine. How does having more power make the car behave differently? What does power mean?What units are used to measure power?