Download Work and Energy - ICP-Physics, Ms. Ave, PHHS

Work and Energy
1. What is work?
2. What is energy?
3. What does horsepower and torque of
an engine tell you about a car?
Work
Work is a force exerted for a certain
distance.
Work = Force * Distance
W=Fd
Examples of work
Lifting a book over your head
Walking up the stairs
Work
Example of no work
Holding a book up, but not moving it.
You may exert energy holding the book up, but if it
doesn’t move no work is being done.
A ball rolling along the ground
No force is applied to keep the ball moving so no work
is being done.
Pushing against a wall
You may be exerting a force on the wall, but the wall
does not move so no work is being done.
Work is measured in…
W=F*d =Newton * meters =Joules
Power
Power is the rate at which work is done.
Power = work/time
P=W/t
Power is measured in…
P=W/t
Joules/second = Watt
Practice Problems
How much work is required to lift a
10 N weight 5 m in 4 seconds?
m= 10N
d= 5m
W=10N * 5m
W=Fd
W=50J
W=50J
P=50J/4s
P=W/t
P=12.5w
How much power was consumed in the
previous question?
t= 4s
Practice Problems
How high would a 10 kg mass need to
be lifted so that a student does 500J
of work?
M=10kg
W=500J
F=ma 10kg*10N/kg
d=500J/100N
d=5m
d=W/F
F=100N
Practice Problems
How much time would it take the
student to lift the 10kg mass from
the previous question so that he uses
1,000watts of power?
W=500J
P=1,000w t=W/P
t=500J/1,000w
t=.5s
Homework Time
Bell Work
What is work?
What is power?
Define energy?
How do you get energy?
Mechanical Energy
What is energy?
The ability to do work.
Energy comes in many forms: heat,
light, sound, electricity, radioactivity
or even mass (E=mc2).
All energy is measured in joules
Potential and Kinetic Energy are the 2
types of Mechanical Energy
Potential Energy
PE is stored energy.
If an object has potential energy, that
object has the potential to do work.
PE could be stored in a rubber band,
springs, chemically (gun powder), by
gravity (lifting an object over your
head),etc.
Gravitational Potential Energy = weight * height
GPE = mgh
Kinetic Energy
KE is energy in motion
Roller coaster Lab
Bell Work
What is PE?
What is KE?
Roller Coaster Physics
Where does the skater have the most
GPE?
Top
Why?
Highest point
How much GPE does he have here?
GPE=mgh
GPE=50kg*10N/kg*6.40m =3200J
Roller Coaster Physics
Where does the skater have the most
Kinetic Energy?
Bottom
How much?
Looking at the bar graph, it should be
equal to the amount of PE when it was at
the top: 3200J
Roller Coaster Physics
Law of Conservation of Energy
Energy cannot be created nor destroyed.
It can only change forms
True for frictionless simulations, but
what about when friction is on?
Simulation
Yes, it just changes into thermal (heat)
energy.
Roller Coaster Physics
If a chain were to pull them up to
that high point (from the blue dotted
line), how much work did the chain
do?
W=Fd
F=weight
D=6.4m
50kg*10N/kg
W=500N*6.4m=3200J
Work Energy Theorem
If you apply net force over a distance that
is work
If you apply a net force that changes the
velocity of an object, there has been a
change in KE
Work Energy Theorem:W= ΔKE
Can also work for ΔPE
Homework Time
Kinetic Energy
KE is energy in motion
KE=1/2 mv2
Which is Kinetic Energy more
dependent on: mass or velocity?
Velocity
If you double the mass, KE is doubled.
If you double the velocity, KE is
quadrupled.
Practice Problems
How many joules of work are done
when a force of 4 N moves a book 3
m?
F=4N d=3m
W=4N*3m
W=F*d
W=12J
Practice Problem
How many watts of power are
expended when a force of 20N moves
a block 6m in 3s?
F=20N
d=6m
W=F*d
W=20N*6m
P=120J/3s
t=3s P=W/t
W=120J
P=40watts
Practice Problem
How much PE does a 14kg mass have
after lifting it to a height of 3m in 20
seconds?
PE=mgh
M=14kg
d=3m
g=10m/s/s
PE= 14kg*10m/s/s*3m PE=420J
Practice Problem
How much KE does an 8kg mass
traveling at a velocity of 5m/s?
M=8kg
v=5m/s
KE=1/2*8kg*52
KE=1/2 mv2
KE= 100J
Bell Work
1.
Which of the following has more energy?
A ball with a mass of 4kg and a velocity
of 2m/s, or a ball with a mass of 2 kg and
a velocity of 4m/s?
2. Which has a greater KE, a 100 ton ship
docked (meaning its motionless) or a
feather falling through the air?
3. How much work is done when I lift a 10N
mass 4m? How much PE does the mass
have?
Work Energy Theorem
What is an acceleration?
Change in velocity
What causes an acceleration?
Force
If you apply force over a distance that is
work
If you apply a net force that changes the
velocity of an object, there has been a
change in KE
Work Energy Theorem:Work= ΔKE
Can also work for ΔPE
Work Energy Theorem
Work Energy Theorem applies to an
increase and a decrease in velocity.
Applying the brakes does work.
If the change in KE doubles, the work
done by the brakes doubles.
Fd= ΔKE
d= 1/2 m(vf-vi)2 / F
Practice Problem
Using d= 1/2 mv2 / F if a car has a
mass of 3,500kg, a velocity of 13m/s
(30 mph) and can apply a braking
force of 30,000N, what will the
stopping distance be?
d=½ * 3,500*132/30,000
d=10m (33ft)
Practice Problem
If you were to double your velocity
from 13 to 26 m/s (60mph) what do
you predict the stopping distance will
be? Calculate the actual stopping
distance.
d=½ * 3,500*262/30,000
d=40m (132 ft)
Stopping Distance and speed
If you double the velocity, the
stopping distance is quadrupled. If
you triple the velocity, the stopping
distance goes up 9 times!!!
In other words, a small increase in
speed causes the stopping distance to
increase greatly.
Stopping Distance and speed
In school zones, the speed limit is
9m/s (20mph). Using the same car as
before, what is the difference in
stopping distance if you travel at
13m/s(30mph)?
d=½ * 3,500*92/30,000
d=4.7m
Difference: 10-4.7=5.3m =17.5ft
Conservation of Energy
Energy cannot be created or
destroyed. Energy can change from
one form or another, but the total
amount of energy NEVER changes.
Roller Coaster Physics
Practice Problem
A roller coaster at the bottom of the hill
has a mass of 400kg and a velocity of
7m/s. It climbs to the top of the next hill
and comes to a complete stop before
rolling backwards. How much PE does it
have at the top?
KE at the bottom = PE at the top
KE= 1/2 mv2
½ *400kg *72
KE=9800j=PE
Practice Problem
A car traveling at 40 km/h can stop in
12m. If the same car is traveling at
160km/h, how far will the car skid to
a stop?
4 times the velocity means stopping
distance = 42 * the original stopping
distance = 12 * 16 = 192 m