Transcript Power

12/6 do now
• Is conservation of mechanical energy likely to
hold in this situation?
• An ice skater glides across freshly made ice.
Explain your reasoning.
5.3 Conservation of energy – due
Monday 12/2
1. What is the meaning of the word “conserved”? Give
example(s) of some quantity that are conserved.
2. What is Mechanical Energy? Is it a new form of
energy? What are some forms of non-mechanical
energy?
3. When is mechanical energy conserved?
4. When an object encounters friction, its mechanical
energy is lost. What does mechanical energy become?
5. Is total energy conserved even when mechanical
energy is not conserved?
objectives
1. Relate the concepts of energy, time, and power.
2. Calculate power in two different ways.
3. Explain the effect of machines on work and power.
• Lab 10 – energy of a tossed ball
Homework:
• 5.4 essay – due Mon 12/9
• Castle Learning – extra credit on test if
everyone does the castle learning
assignment
questions
• Joe and Joey weights the same and both run
up the stairs. Joe runs in 30 second and Joey
runs in 20 second.
• Who does more work? What is different?
Power
• Power is the rate at which work is done or the rate of energy
transfer by any method. It is the work/time ratio.
• Mathematically, it is computed using the following equation.
• The standard metric unit of power is the Watt.
• Example:
• A 60 watt light bulb transfers 60 J every second of electrical
energy into light energy.
• All machines are typically described by a power rating. The power
rating indicates the rate at which (how fast) that machine can do
work upon other objects. Machines with different power rating do
the same work in different time intervals. A 12 watt vacuum can do
the job 2 times faster than a 6 watt vacuum.
• The power rating of a car relates to how rapidly the car can be
accelerated.
• The power of a light bulb indicate how fast electrical energy can be
transferred to light energy. A 60 watt light bulb transfers 60 J of
energy per second. A 100 watt light bulb transfers 100 J of energy
per second.
• Some people are more power-full than others. That is, some people
are capable of doing the same amount of work in less time or more
work in the same amount of time.
Question
• Many mountain roads are built so that they
zigzag up the mountain rather than go straight up
toward the peak. What is the advantages of such
a design from the view point of energy
conservation and power?
• Assuming mechanical energy is conserved, the
same amount of energy is needed to reach the
top in both cases. Because the same amount of
work must be done, the path with a longer
distance takes more time and hence requires less
power.
Example 1
• Ben Pumpiniron elevates his 80-kg body up the 2.0meter stairwell in 1.8 seconds. What is his power?
It can be assumed that Ben must apply an (80 kg x 9.81 m/s2) Newton downward force upon the stairs to elevate his body.
Example 2
•
Two physics students, Will N. Andable and Ben
Pumpiniron, are in the weightlifting room. Will lifts
the 100-pound barbell over his head 10 times in
one minute; Ben lifts the 100-pound barbell over
his head 10 times in 10 seconds. Which student
does the most work? ______________ Which
student delivers the most power? ______________
Explain your answers.
Example 3
• When doing a chin-up, Julia lifts her 42.0-kg body a
distance of 0.25 meters in 2.0 seconds. What is the
power delivered by Julia’s biceps?
Example 4
• A 193 kg curtain needs to be raised 7.5 m, at
constant speed, in as closed to 5.0 s as
possible. The power ratings for three motors
are listed as 1.0 kW, 3.5 kW, ad 5.5 kW. Which
motor is best for the job?
P 
W
t

Fd cos 

mgd cos 0
t
t
2
P 
(193 kg )( 9 . 81 m / s )( 7 . 5 m )( 1)
5 .0 s
 2 . 8 kW
Another equation for power
P
W
t

F  d  cos 
t
 F  v  cos 
kilowatt-hour is unit for energy
• Your household's monthly electric bill is often
expressed in kilowatt-hours. One kilowatt-hour is
the amount of energy delivered by the flow of l
kilowatt of electricity for one hour. Use conversion
factors to show how many joules of energy you get
when you buy 1 kilowatt-hour of electricity.
12/11 do now
• A 1.00 × 103 kg sports car accelerates from
rest to 25.0 m/s in 7.50 s. What is the average
power output of the automobile engine?
P 
W
P  F  v avg
t
W   KE
F  ma  m
v
t
12/12 do now
• Water flows over a section of Niagara Falls at a
rate of 1.20 × 106 kg/s and falls down a height
of 50.0 m. What is the power of the waterfall?
Objectives
• Work, energy and power review
• Homework – castle learning assignments – 4
new assignments – due Fri.
• Chapter test is on Friday
Class work
Page 189
Practice 5F #1-5
1. 65 kW
2. 2.38 x 104 W
3. 2.61 x 108 s
4. 3.6 x 103 s
5. a. 7.50 x 104 J;
b. 2.50 x 104 W
Page 189-Section Review #1-3; Page 195 #35-36
2.
3.
35.
36.
12.3 s; 2450 J
613 W; 2450 J
17.2 s
5.9 x 108 W
Page 193
#1-6
pp. 193-194
#11-18
Page 194
#26-32
Lab 9 – forces on a spring
• Finish – fix you lab in your lab group
Lab 10 - Energy of a Tossed Ball
OBJECTIVES
1. Measure the change in the kinetic and potential energy as a ball moves
up and down in free fall.
2. Graph potential energy, kinetic energy, and total energy.
3. Analyze the graph to determine how much kinetic energy is lost.
4. Reach conclusions regarding the amount of energy possessed by the ball
as it moves in air.
MATERIALS
• Ball, Computer with Vernier Program, Interface, motion detector, basket
PRELIMINARY QUESTIONS
• For each question, consider the free-fall portion of the motion of a ball
tossed straight upward, starting just as the ball is released to just before it
is caught. Assume that there is very little air resistance.
1. What form or forms of energy does the ball have while
momentarily at rest at the top of the path?
2. What form or forms of energy does the ball have while in motion
near the bottom of the path?
3. Sketch a graph of position vs. time for the ball.
4. Sketch a graph of velocity vs. time for the ball.
5. Sketch a graph of kinetic energy vs. time for the ball.
6. Sketch a graph of potential energy vs. time for the ball.
7. Sketch a graph of total energy vs. time for the ball.
8. If there are no frictional forces acting on the ball, how is the
change in the ball’s potential energy related to the change in
kinetic energy?
To Nick