Transcript WORK and ENERGY - Cloudfront.net

WORK and ENERGY
Work
Kinetic Energy
Work Energy Theorem
Potential Energy
Conservation of Energy
Power
WORK
Work is a transfer of energy
Force causing an object to have a
displacement
Maximum work: F and d are parallel
Minimum work: F and d are perpendicular
W = Fd
Units: N∙m
SI Units: Joules (J)
Scalar
Independent of pathway
http://www.nu.ac.za/physics/1M2002/Energy%20work%20and%20power.htm
WORK
Importance of sign
+ W: F and d are in the same direction
- W: F and d are in the opposite direction
http://www.physics.upenn.edu/courses/gladney/phys150
WORK
Hewitt Physics
WORK
Graphing Work
Area under the curve of the F vs. d graph.
WORK
Independent of pathway
ENERGY
Ability to do work
Two types


Kinetic Energy
Potential Energy
Heat measures the transfer of energy.
KINETIC ENERGY
Energy of motion
KE = ½ m v2
Units: kg m2 / s2 = Joule
Scalar
Work-Energy Theorem




Net work is equal to the change in energy
W =  KE
Fd = KEf - KE°
Fd = ½ m vf2 - ½ m v°2 = ½ m (vf2 - v°2)
Pg 184 #1-22
1) No
2) Yes +; no; Yes +; yes –
3) No; force decrease,
distance increase; work
constant
4) Tension and Weight are
perpendicular so no
work
5) Longer skids are
moving faster
6) Yes; no; yes; weight and
air resistance
7) 53 J; - 53 J
8) 2.4 x 105 J
9) 47.5 J
10) 6230 J; - 6230 J; 0.640
11) no, yes, yes
12) no; mass is + and v is
squared
13) yes; depends on the
distance
14) 1 to 25
15) No work; motion and
force are perpendicular
16) Speed double; work is
quadrupled…140 m.
17) To climb work must be
done against gravity.
18) Work done by friction
equals particle speeds
POTENTIAL ENERGY
Store energy
Able to do work later
Units: kgm2/s2 = Joules
Scalar
Two main types
Gravitational potential energy
 Elastic potential energy

GRAVITATION POTENTIAL ENERGY
Energy possessed by object
because of its position in a
gravitational field.
W = Fd
PE = mgh
Zero Gravitation Potential Energy
is the point of reference
Pg 172 Sec Rev; pg 185 23-25
4.4 x 10-3 J
2.8 m/s
6.18 x 10-2 J
Kinetic;
nonmechanical;
kinetic, gravitational;
elastic potential
5) Answers vary
1)
2)
3)
4)
23) a) 5400 J, 0 J, 5400 J
b) 0 J, -5400 J; 5400 J
c) 2700 J, -2700 J,
5400 J
24) –19.6 J; 39.2 J; 0 J
25) 0.400 J; 0.225 J; 0J
ELASTIC POTENTIAL ENERGY
Potential energy stored in the
deformation (compression or stretched)
of an elastic object.
Hooke’s Law


Restoring force
F = -kx
W = Fd
PE = ½ k x2
Units: kg m2 / s2 = Joule
CONSERVATION OF ENERGY
First Law of Thermodynamics
Energy is never created or loss; it is just
transfer from one form to another.
Energy before = energy after in an isolated
system.
Second Law of Thermodynamics
Transfer of energy
Mechanical Energy is the total energy
TE = KE + PE (conserved)
TE = KE + PE + Wf (not conserved)
CONSERVATION OF ENERGY
Energy is never created or loss; it is just
transfer from one form to another.
Energy before = energy after in an isolated
system.
Mechanical Energy is the total energy
TE = KE + PE (conserved)
TE = KE + PE + Wf (not conserved)
Pg 186 #26-34, 37
26) nonmechanical;
mechanical; mechanical;
mechanical; both
27) KE Max at bottom
PE Max at top; KE becomes
PE going up and PE
becomes KE going down
28) Conservation of energy
and will not hit the
instructor; give a push
then it will hit
29) Does work on it to move
up; no work, d = 0; does
negative work against
gravity
30) KE max at the bottom;
PE max at the top
31) No, energy is not
conserved
32) Gravitational and
elastic; conservation of
energy.
33) 12.0 m/s
34) 10.9 m/s; 11.6 m/s
37) 0.633 J; 0.633 J; 2.43
m/s; 0.422 J, 0.211J
CONSERVATION OF ENERGY
Hewitt Physics
CONSERVATION OF ENERGY
Hewitt Physics
NEXT-TIME QUESTION
Three baseballs are
thrown from the top of
the cliff along paths A, B
and C. If their initial
speeds are the same and
there is no air resistance,
the ball that strikes the
ground below with the
greatest speed will follow
path
NEXT-TIME QUESTION
If two balls start simultaneously with the some initial speed, the ball to
complete the journey first is along
If the initial speed equals 2 m/sec, and the speed of the ball at the bottom of the
curve on Track B is 3 m/sec, then the speed of the ball at the top of the curve on
Track A is
POWER
Rate work is done or which energy is
transferred
P=W/t
= Fd / t
=Fv
Units: J/s = Watts (W)
CONCEPTS
As a consultant to the soft-drink industry, Dr. J is given the task of conducting the ultimate Pepsi
taste test. This is Dr. J's tenth taste test, which puts him seven up on his nearest consultant, who
had only done three. Of course Dr. J is very qualified, having been hooked on soft drinks
(especially orange soda) since he was Nehi to a pop bottle. Dr. J mounts a rather large container
of Pepsi on a ledge some 3 meters above the ground. A bullet of mass 5 grams is then fired into
the container, thus killing the taste. Not only that, but the Pepsi falls through the bullet hole onto
the ground below (causing the taste to go flat). The wall of the container is 2 cm thick. The velocity
of the bullet changes from an initial value of 500 m/sec just before striking the container wall to 5
m/sec upon leaving the container wall and entering the Pepsi. It finally fizzles out at a point 25 cm
from the container wall.
A. How much work does the container wall do on the bullet?
How much work does the Pepsi do on the bullet?
At what velocity does the Pepsi hit the floor?
Chapter 10: R pg 199
1) A force of 825 N is needed to push a car
across a lot. Two students push the car
35m.
a) How much work is done?
b) After a rainstorm, the force needed to
push the car doubled because the ground
became soft. By what amount does the
work done by the students change?
29000J; work doubles
Chapter 10: R pg 199
2) A delivery clerk carries a 34 N package
from the ground to the fifth floor of an
office building, a total height of 15 m. How
much work is done by the clerk?
510 J
3) What work is done by a forklift raising a
583 kg box 1.2 m?
6900 J
Chapter 10: R pg 199-202
4) You and a friend each carry identical boxes to
a room one floor above you and down the hall.
You choose to carry it first up the stairs, then
down the hall. Your friend carries it down the
hall, then up another stairwell. Who does more
work?
Same amount of work
5) How much work does the force of gravity do
when a 25 N object falls a distance of 3.5 m?
88 J
Chapter 10: R pg 202
6) An airplane passenger carries a 215 N
suitcase up stairs, a displacement of 4.20 m
vertically and 4.60 m horizontally.
a) How much work does the passenger do?
b) The same passenger carries the same
suitcase back down the same stairs. How much
work does the passenger do now?
903 J; -903 J
7) A rope is used to pull a metal box 15.0 m
across the floor. The rope is held at an angle of
46.0 ° with the floor and a force of 628 N is
used. How much work does the force on the
rope do?
6540 J
Chapter 10: R pg 202-203
8) A worker pushes a crate weighing 93 N
up an inclined plane, pushing horizontally,
parallel to the ground in the figure.
a) The worker exerts a force of 85 N. How
much work does he do?
b) How much work is done by gravity?
c) The coefficient of friction is = 0.20.
How much work is done by friction?
340 J; -279 J; 130 J
Answers: Chapter 10
1) 800 J
2) 12000 J
3) 59.9 kg
4) 1.86 x 105 J
5) 0.80 J
6) 25 N/m; 0.50 J
7) 600 J
8) 826 J; 1.13 x 10 4 J; - 1.13 x 10 4 J
9) 1.20 x 10 4 J
10) 58.7 degrees
11) 1.8 x 10 4 J
12) no work
13) 7.7 J
14) 518 J
Chapter 10: R pg 202-203
9) A box that weighs 575 N is lifted a
distance of 20.0 m straight up by a rope.
The job is done in 10.0 s. What power is
developed in watts and kilowatts?
1150 W; 1.15 kW
Chapter 10: R pg 203
10) A rock climber wears a 7.50 kg
knapsack while scaling a cliff. After 30.0
min, the climber is 8.2 m above the
starting point.
a) How much work does the climber do on
the knapsack?
b) If the climber weighs 645 N, how much
work does she do lifting herself and the
knapsack?
c) What is the average power developed
by the climber?
600 J; 5900 J; 3.3 W
Chapter 10: R pg 203
11) An electric motor develops 65 kW of
power as it lifts a loaded elevator 17.5 m in
35.0s. How much force does the motor
exert?
1.3 x 105 N
Chapter 10: R pg 203
12) Two cars travel the same speed, so that they move
105 km in 1 h. One car, a sleek sports car, has a motor
that delivers only 35kW of power at this speed. The
other car needs its motor to produce 65 kW to move the
car this fast. Forces exerted by friction from the air
resistance cause the difference.
a) For each car, list the external horizontal forces exerted
in it, and give the cause of each force. Compare their
magnitudes.
b) By Newton’s third law, the car exerts forces. What are
their directions?
c) Calculate the magnitude of the forward frictional force
exerted by each car?
d) The car engines did work. Where did the energy they
transferred come from?
Road on car; air on car; 1200 N; 2200N; chemical
Answer Chapter 10: pg214
15) 7400J
16) 800 J; 600 J
17) -5.53 x 10 3 J; no work; 5.53 x 10 3 J; no; 2.2
kW
18) 9000 J; 3.00 kW
19) 348 W; 696 W
20) 220 J; 110W
21) 110 kJ; 3.14 kW
22) 1.8 x 10 4 J; 2.3 kW
23) 160 W
Answer Chapter 10: pg214
24) 54.7 m
25) 368 W
26) 90 kW
27) 2890 N
28) 2300 N
Chapter 10: R pg 203
12) Two cars travel the same speed, so that they move
105 km in 1 h. One car, a sleek sports car, has a motor
that delivers only 35kW of power at this speed. The
other car needs its motor to produce 65 kW to move the
car this fast. Forces exerted by friction from the air
resistance cause the difference.
a) For each car, list the external horizontal forces exerted
in it, and give the cause of each force. Compare their
magnitudes.
b) By Newton’s third law, the car exerts forces. What are
their directions?
c) Calculate the magnitude of the forward frictional force
exerted by each car?
d) The car engines did work. Where did the energy they
transferred come from?
Road on car; air on car; 1200 N; 2200N; chemical