dr. joseph w. howard ©spring 2008 energy what is it? it is how nature keeps score. like a...
TRANSCRIPT
Dr. Joseph W. Howard
©Spring 2008
EnergyEnergyWhat is it?
It is how nature keeps score.
Like a “currency” of the universe.
To cause a change requires energy
Dr. Joseph W. Howard
©Spring 2008
First StepsFirst Steps
How do we “measure” effort?
How do you get paid ($$$) in the world?
Job! Work!($$$)
Dr. Joseph W. Howard
©Spring 2008
Work & EnergyWork & Energy
Technical Definition of Work
WORK = FORCE DISTANCE = mad
ENERGY IS THE CAPACITY TO DO WORK.
It takes some work (energy) to cause a change.
net
Dr. Joseph W. Howard
©Spring 2008
Work ExampleWork Example
5m
A donkey pulls a 10 kg box a distance of 5 m by applying a 90 N net force. How much work has the donkey done?
A donkey pulls a 10 kg box a distance of 5 m by applying a 90 N net force. How much work has the donkey done?
90N
10kg
Work = (90 N) × (5 meters)
Work = 450 N m = 450 Joulesg
Work = Net Force × distance
Dr. Joseph W. Howard
©Spring 2008
10m
3m/s
Hoyt carries a very heavy boulder 10m across the garden at a constant speed of 3 m/s. What is the overall work done on the boulder?
Hoyt carries a very heavy boulder 10m across the garden at a constant speed of 3 m/s. What is the overall work done on the boulder?
Conceptual PitfallConceptual Pitfall
Work = ZERO!!Work = ZERO!!
Dr. Joseph W. Howard
©Spring 2008
Types of EnergyTypes of Energy
KE is often thought of as energy of motion
PE is often thought of as energy of position
KE is often thought of as energy of motion
PE is often thought of as energy of position
Kinetic & Potential EnergiesKinetic & Potential Energies
Dr. Joseph W. Howard
©Spring 2008
Gravitational PEGravitational PE
A 10 kg ball rests at the top of a set of stairs. The stairs reach a height of 3 m above the ground. What is the potential energy of the ball?
A 10 kg ball rests at the top of a set of stairs. The stairs reach a height of 3 m above the ground. What is the potential energy of the ball?
. .P E N m Joules 294 294g
. .P E mgh
. . ( ) . ( )m
P E kg ms
210 9 8 3
Dr. Joseph W. Howard
©Spring 2008
Kinetic EnergyKinetic EnergyAn oxygen atom has a mass of 2.66 10-23 g. If an oxygen atom were moving at 200 m/s, what would the kinetic energy of that atom be?
An oxygen atom has a mass of 2.66 10-23 g. If an oxygen atom were moving at 200 m/s, what would the kinetic energy of that atom be?
. . .m
K E kgs
2261
2 66 10 2002
. .K E mv 212
. . .K E Joules 225 32 10
Dr. Joseph W. Howard
©Spring 2008
Kinetic EnergyKinetic EnergyA 3-kg ball is rolling at a constant speed. If you had to transfer 30 J of energy to the ball to cause this motion, what must the velocity of the ball be?
A 3-kg ball is rolling at a constant speed. If you had to transfer 30 J of energy to the ball to cause this motion, what must the velocity of the ball be?
mv
s
22
220
. .K E mv 212
.J kg v 2130 3 0
2
( ).J
vkg
2 2 303 0
( ).N m
vkg
2 2 303 0
( )
.
mkg msvkg
22
2 30
3 0
mv
s
22
220 .m
vs
4 5
Dr. Joseph W. Howard
©Spring 2008
Law of Conservation of Energy
Law of Conservation of Energy
Total energy in any process is constant.
The energy may be transferred or transformed,
but not created nor destroyed.
Total energy in any process is constant.
The energy may be transferred or transformed,
but not created nor destroyed.
Dr. Joseph W. Howard
©Spring 2008
Law of Conservation of EnergyLaw of Conservation of Energy
Total Energy Total Energy at the at the
beginning of beginning of an eventan event
Total Energy Total Energy at the at the
beginning of beginning of an eventan event
=Total Energy Total Energy at the end of at the end of
an eventan event
Total Energy Total Energy at the end of at the end of
an eventan event
Form of energies my change
Dr. Joseph W. Howard
©Spring 2008
Conservation of EnergyConservation of EnergyConsider a 20-kg ball rolling down a hill that is 5 m high. How fast will the ball be moving when it reaches the bottom of the hill?
Consider a 20-kg ball rolling down a hill that is 5 m high. How fast will the ball be moving when it reaches the bottom of the hill?
What energy here?What energy here?
What energy What energy here?here?
The PE energy at the top becomes the
KE energy at the bottom
top top bottom bottomKE PE KE PE
( ) . ( )top
mPE kg m Joules
s 220 9 8 5 980
( )
bottomJoules KE
Joules kg v
2
980
1980 20
2
.m
vs
9 95m
Dr. Joseph W. Howard
©Spring 2008
Conceptual PitfallConceptual PitfallA young boy skates down each of the frictionless playground ramps illustrated below. Which ramp will give the skateboarding boy the fastest speed at the bottom of the ramp?
A young boy skates down each of the frictionless playground ramps illustrated below. Which ramp will give the skateboarding boy the fastest speed at the bottom of the ramp?
h
Ramp A Ramp B Ramp C Ramp D
Any Ramp, all ramps will produce the same speed at the bottom.
Ramp A Ramp B Ramp C Ramp D
Any Ramp, all ramps will produce the same speed at the bottom.
A
B CD
Dr. Joseph W. Howard
©Spring 2008
Law of Conservation of EnergyLaw of Conservation of Energy
Total Energy Total Energy at the at the
beginning of beginning of an eventan event
Total Energy Total Energy at the at the
beginning of beginning of an eventan event
=Total Energy Total Energy at the end of at the end of
an eventan event
Total Energy Total Energy at the end of at the end of
an eventan event
Form of energies my change
Dr. Joseph W. Howard
©Spring 2008
Conservation of EnergyConservation of EnergyA 5 kg bowling ball falls from rest a distance of 78.4 m. How fast is the ball falling at that point?A 5 kg bowling ball falls from rest a distance of 78.4 m. How fast is the ball falling at that point?
What energy here?What energy here?
What energy here?What energy here?
78.4m. ( . )J kg v 23841 6 2 5
mgh mv 212
top top bottom bottomKE PE KE KE
( )( . )( . ) ( )m
kg m kg vs
22
15 9 8 178 4 5
2
.m
vs
2
221536 6
.m
vs
39 2
Dr. Joseph W. Howard
©Spring 2008
Conservation of EnergyConservation of EnergyConsider dropping a ball from a height of 15 m in a vacuum. What is the velocity of the ball the instant it strikes the ground?
Consider dropping a ball from a height of 15 m in a vacuum. What is the velocity of the ball the instant it strikes the ground?
What energy here?What energy here?
What energy here?What energy here?
15m
The PE energy at the top becomes the
KE energy at the bottom( . )( )
mv m
s2
2
19 8 15
2
mgh mv 212
top top bottom bottomKE PE KE PE
( . )( )m
m m mvs
22
19 8 15
2
( . )( )m
v ms
222 9 8 15
.m
vs
17 1
Dr. Joseph W. Howard
©Spring 2008
A 5kg ball is rolling along a sidewalk with a constant velocity of 3 m/s. Suddenly, the ball encounters a 0.5 m dip in the sidewalk and then continues rolling along a flat section of the sidewalk. What will the velocity be for the ball after it encounters the dip?
A 5kg ball is rolling along a sidewalk with a constant velocity of 3 m/s. Suddenly, the ball encounters a 0.5 m dip in the sidewalk and then continues rolling along a flat section of the sidewalk. What will the velocity be for the ball after it encounters the dip?
Conservation of EnergyConservation of Energy
What energy here?What energy here?
What energy here?What energy here?
Dr. Joseph W. Howard
©Spring 2008
A
B
a. Ab. Bc. same
Tracks A and B are of equal length and have hills of the same curvature except A curves up and B curves down. If two identical balls are rolled simultaneously with the same initial velocity, which will reach the end of its track first?
Tracks A and B are of equal length and have hills of the same curvature except A curves up and B curves down. If two identical balls are rolled simultaneously with the same initial velocity, which will reach the end of its track first?
Conceptual PitfallConceptual Pitfall
Dr. Joseph W. Howard
©Spring 2008
Energy & MotionEnergy & MotionA 10kg dog runs from a speed of 4 m/s up to a speed of 10 m/s in 3 seconds. What is the dog’s acceleration? How far does he run in those 3 seconds?
How much did the dog’s kinetic energy change?
How much work did the dog accomplish?
A 10kg dog runs from a speed of 4 m/s up to a speed of 10 m/s in 3 seconds. What is the dog’s acceleration? How far does he run in those 3 seconds?
How much did the dog’s kinetic energy change?
How much work did the dog accomplish?
Dr. Joseph W. Howard
©Spring 2008
Two pumpkins of equal size and mass are dropped off the roof of the Henson Science Hall. One lands on the sidewalk and the other lands on the grass. Which one of the following statements is true of the force and impulse on the pumpkins as their fall was stopped?
• Both of the pumpkins experienced the same force and the same impulse.
• Both of the pumpkins experienced the same impulse, but the pumpkin that hit the sidewalk experienced a greater force.
• Both of the pumpkins experienced the same force, but the pumpkin that hit the grass experienced a smaller impulse.
• The pumpkin that hit the sidewalk experienced a greater force and a greater impulse than the pumpkin that hit the grass.
• The relationship between the impulses and forces on the pumpkins can not be determined without knowing the height of the building.
• Both of the pumpkins experienced the same impulse, but the pumpkin that hit the sidewalk experienced a greater force.
Two water slides sit side by side at the water amusement park and these both sit near a high dive tower, as shown below. Your annoying little brother wants to do the ride that promises the fastest attainable speed at the bottom. What recommendation can you make to your brother about which ride should he should choose? Assume that there is no friction on the slides and that air resistance is negligible.
High dive Water slide B Water slide A
Water slide B or the High Dive Any of the rides
Water slide A High DiveWater slide B