1. Work, Power, ENERGY

2. QOTD: Write a list of 10 examples your idea of work.
Demo : create a work equation using a spring scale, string and a book.

3. What is work? Work is done only when a force moves an object
A force acting on an object and causing it to move a distance is work
Not every force is work..if you push against the wall it does not move..that is not work!
Work = force X distance
Work is measured in Joules
If you pick up a bag of groceries and walk across the room the work is picking up the groceries not the walking.

4. Ie: the groceries You walk
The object must move some distance as a result of your force
The force you exert must be in the same direction as the objects motion.
Ie: the groceries
You walk

5. What’s work?
A scientist delivers a speech to an audience of his peers.
A body builder lifts 350 pounds above his head.
A mother carries her baby from room to room.
A father pushes a baby in a carriage.
A woman carries a 20 kg grocery bag to her car?

6. What’s work?
A scientist delivers a speech to an audience of his peers. No
A body builder lifts 350 pounds above his head. Yes
A mother carries her baby from room to room. No
A father pushes a baby in a carriage. Yes
A woman carries a 20 kg grocery bag to her car? No

7. Work can be determined by calculating
Force used x distance moved = amount of work
Therefore what is the formula for work?

8. Work = force x distance
 Joule – is the SI unit for work.
Newton = force
Meters = distance
  Therefore if you exert:
1 Newton of force for 1 meter of distance = 1 joule of work or 1N/m

9. Work is done when a force is exerted through a distance.
A student lifts a bag of books that weighs 135 N. If the bag is lifted .75 m, how much work does the student do?
F = 135 N d = .75 m
W = Fd
W = (135 N)( .75 m)
W = J

11. Negative Work
If the object moves in the direction opposite the direction of the force
(for instance if force and displacement are in opposite directions)
then the work is negative: W < 0.
The energy of the system is reduced.

13. A +24 N force is applied to an object that moves 10 m
in the same direction during the time that the force is
applied. How much work is done to the object?

14. http://www2. franciscan

15. Force-N distance -m time- s
Find the equation for POWER
1. Attach a string and spring scale to a large book.
2. Pull the book .5m slowly. Use a stopwatch to determine time .
3. Record the time and distance on a data table.
4. Repeat 1- 3 – but this time pull the book faster.
5. Repeat 1-3 even faster.
Force-N distance -m time- s

16. Power
Power tells you how fast something is happening..how fast the work is being done
Power = work/time or Power = Force X Distance
Time
Power is measured in watts (W)
One watt is equal to 1 joule per second of work divide joules/seconds

17. Power – the rate at which energy is transferred.
P = W P = power Watts
t W = work Joules
t = time seconds
1 Watt (W) = 1 J/s

18. How much power must a motor have to operate a pump that raises kg of water every minute a distance of 12 m?

19. 1. m = 1500 kg t = 60 s d = 12 m
 2. Equations-
P = W/t W = Fd F = mg
 3. Plug and chug-
F = mg = (1500 kg)(9.8 m/s2) = 14,700 N
W = Fd = (14,700 N)(12 m) = 1.76 x 105 J
P = W/t = (1.76 x 105 J)/(60 s) = 2940 W

21. What is Energy?
It turns out that energy is so fundamental, like space and
time, that there is no good answer to this question. However,
just like space and time, that doesn't stop us from doing very useful calculations with
Energy
We may not be able to define energy, but because it is a
conserved property of nature, it's a very useful idea.

22. Potential Energy Potential Energy (PE): Stored energy due to position
Examples: rock on a cliff, battery, food, gasoline, stretched rubber band, apple hanging in a tree

23. Gravitational Potential Energy
A barbell of mass "m" is lifted vertically upwards a distance 
"h" by an outside force. How much work does that outside 
force do on the barbell?
Fapp mg
W = Fdparallel
 Since a = 0, Fapp = mg
W = (mg) dparallel
 Since F and d are in the same   direction ...and dparallel = h
W = (mg) h
W = mgh

24. Gravitational Potential Energy
But we know that in general,
Eo + W = Ef.
If our barbell had no energy to 
begin with, Eo = 0, then W = Ef
But we just showed that we did W=mgh to lift the barbell... so mgh=Ef
The energy of a mass is increased by an amount mgh when it is raised by a height "h".

25. Gravitational Potential Energy
The name for this form of energy is
Gravitational Potential Energy (GPE).
GPE = mgh
One important thing to note is that while changes in gravitational 
potential energy are important, their absolute value is not.

26. Gravitational Potential Energy
0 m
0.5 m
You can define any height to 
be the zero for height...and 
therefore the zero for GPE.
But whichever height you 
choose to call zero, changes 
in heights will result in 
changes of GPE. For 
example, the floor level can 
be considered zero energy or 
the ladder level can be zero.

27. Gravitational PE Gravitational PE (GPE):
Energy stored by objects that are above the earth’s surface (objects that can fall)
Depends on mass, acceleration and height
GPE increases with height

28. Gravitational PE GPE = mass  gravity  height
GPE = m g h = weight  height
  GPE = m (kg)  9.8 m/s2  h (m)
j = 1 Nm

29. 9
What is the change of GPE for a 5.0 kg object 
which is raised from the floor to a final height of 
2.0m above the floor?
answer
GPE=mgh
GPE= (5kg)(9.8)(2m)
GPE=98 J

30. 10 A increases B decreases C stays the same
As an object falls, its GPE always _____. A
increases B
decreases C
stays the same
answer B

31. 11
What is the change of GPE for a 8.0 kg object which 
is lowered from an initial height of 2.0 m above the 
floor to a final height of 1.5m above the floor?
answer
GPE=mgh
GPE= (8)(9.8)(-0.5)
GPE= J

32. 12
What is the change in height of a 2.0 kg object 
which gained 16 J of GPE?
answer
GPE=mgh
h = GPE/mg
h = 16/(2)(9.8)
h = 0.82m

33. Kinetic Energy Kinetic Energy (KE): Energy in the form of motion
Depends on mass and velocity of moving object.
  Object in motion has ability to do work

34. Kinetic Energy The amount of KE an object has is given by:
The energy an object has by 
virtue of its motion is called its 
kinetic energy. The symbol we will 
be using for kinetic energy is KE.
Like all forms of energy, it is 
measured in Joules (J).
The amount of KE an object has is given by:
KE = 1/2 mv2

35. Kinetic Energy KE = ½ mass  velocity2 KE = m  V2 2
KE = m  V2 2
(j) = (kg)  (m/s)
 1 j = 1 kg m/s

36. 13 decreases increases C As an object falls, its KE always _____.
stays the same.
answer B

38. 14
A ball falls from the top of a building to the ground 
below. How does the kinetic energy (KE) compare to the potential energy (PE) at the top of the building? A
KE = PE B
KE > PE C
KE < PE
answer C D
It is impossible to tell.

39. 15
What is the kinetic energy of a 12 kg object 
with a velocity of 10 m/s?
answer
KE = 0.5 mv2
KE = (0.5)(12)(100)
KE = 600 J

40. 16
What is the mass of an object which has 2400 J 
of KE when traveling at 6.0 m/s?
answer
KE = 0.5 mv2
m = KE / (0.5)(v2)
m = 2400 / (0.5)(62)
m = kg

41. A 3 kg object has 45 J of kinetic energy. What is its velocity?17
A 3 kg object has 45 J of kinetic energy. What is 
its velocity?
answer
KE = 0.5 mv2
v2 = KE/0.5m
v2 = 45 / 0.5 (3)
v = 5.48 m/s

42. 18 quadrupled quartered C D doubled
If the speed of a car is doubled, the KE of the car is: A
quadrupled B
quartered C
halved D
doubled
answer A

43. 19
Which graph best represents the relationship between the KE and the velocity of an object accelerating in a straight line? A KE v C
answer D B D

44. 20
The data table below lists mass and speed for 4 objects. Which 2 have the same KE? A
A and D B
B and D C
A and C D
B and C
answer D

45. Elastic Potential Energy
Energy can be stored in a spring, this 
energy is called Elastic Potential Energy.
Robert Hooke first observed the 
relationship between the force necessary 
to compress a spring and how much the 
spring was compressed.

46. Hooke's Law Fspring = -kx
k represents the spring constant and is measured in N/m.
x represents how much the spring is compressed and is 
measured as you would expect, in meters.
The - sign tells us that this is a restorative force.
(if you let the spring go once it is compressed, it
will go back to its original position)

47. Elastic Potential Energy
The work needed to compress a spring is equal to the area 
under its force vs. distance curve.
Area of a triangle = 1/2 b h
(F= -kx)
W = 1/2 (x)(F)
W = 1/2 (x)(kx)
W = 1/2kx2

48. Elastic Potential Energy
The energy imparted to the spring by this work must be stored 
in the Elastic Potential Energy (EPE) of the spring:
EPE = 1/2 k x2
Like all forms of energy, it is measured in Joules (J).

49. 21
Determine the elastic potential energy stored in a 
spring whose spring constant is 250 N/m and 
which is compressed 8 cm.
answer
EPE = 0.5 kx2
EPE = 0.5 (250)(0.082)
EPE = 0.8 J

50. 22
What is the spring constant of a spring that is 
compressed 5 cm and has 0.65 J of elastic 
potential energy stored in it?
answer
EPE = 0.5 kx2
k = EPE/0.5x2
k = 0.65 / 0.5 (0.052)
k = 520 N/m

51. 23
How much does a spring with a spring constant of 
500 N/m need to be compressed in order to store 
1.75 J of elastic potential energy?
answer
EPE = 0.5 kx2
x2 = EPE / 0.5k
x2 = 1.75 / 0.5(500)
x = 0.08m

52. A 3 kg mass compresses a spring 2.5 cm. What is the spring constant?24
A 3 kg mass compresses a spring 2.5 cm. What is 
the spring constant?
answer
F = ma F = kx
F = 3(9.8) k = F / x
F = 29.4N k = / 0.025
 k = 1176 N/m  

53. k = 1176 N/m 25
The same 3 kg mass compresses the same spring 
2.5 cm. How much elastic potential energy is 
stored in the spring?
answer
EPE = 0.5kx2
EPE = (0.5)(1176)(0.0252)
EPE = J  

55. Law of Conservation of Energy
The law of Conservation of Energy:
Energy cannot be created or destroyed. It may be transformed from one form into another; however, the total amount of energy in the universe remains constant. (Transformers)

56. Law of Conservation of Energy
Energy conversions occur without a gain or loss in energy
Energy into a system = energy out of a system
 Due to friction, energy might seem to be lost, but it has changed into thermal energy. .

57. Energy analogy
When energy is transferred, it can transform (change form) but it still remains energy.
 Analogy:
How is energy like money?
  When money is transferred from one person or place to another it can change form (transform) but it still remains money.

58. Energy Transformations (ball)
Demonstrate: how bounce height of ball becomes lower and lower each time it bounces. Have students infer why this happens.
Each time the ball bounces, part of its energy is transformed into other forms of energy, such as thermal (heat) energy, sound energy and vibrations in the ground. In addition, some energy is absorbed by the ball. Therefore, it will never bounce as high as the initial drop height.

59. Energy Transformations
Ex: A light bulb is a device that transforms electrical energy into electromagnetic (light) energy and thermal energy
Chemical energy (coal) heat energy (burn to create steam) mechanical energy (steam is used to turn turbines) Electromagnetic energy (generates electricity) heat energy (blow drier, oven)

60. Roller Coaster Potential energy becomes Kinetic energy.
PE: 354kJ
KE: 0kJ
V: 0m/s
Potential energy becomes Kinetic energy.
PE: 0kJ
KE: 354kJ
V: 37.1m/S
h=70m
Kinetic energy can become Potential energy.
PE: 0kJ
KE: 354kJ
V: 37.1m/S
PE: 177kJ
KE: 177kJ
V: 26.2m/S
h=35m

61. Conservation of Energy
A roller coaster is at the top of a track that is 80 m high. How fast 
will it be going at the bottom of the hill?
Eo + W = Ef   
Eo = Ef
GPE = KE
mgh = 0.5mv2
v2 = 2gh
v2 = 2 (9.8) 80
v =39.6 m/s 
Conservation of Energy
W = 0
E0 = GPE, Ef = KE
Substitute GPE and KE equations
Solving for v yields

62. A spring gun with a spring constant of 250 N/m is compressed 5 cm
A spring gun with a spring constant of 250 N/m is 
compressed 5 cm. How fast will a kg dart go when it 
leaves the gun?
answer

63. A student uses a spring (with a spring constant of 180 N/m) 
to launch a marble vertically into the air. The mass of the 
marble is kg and the spring is compressed 0.03 m. 
How high will the marble go?
h = 2.066m
answer

64. A student uses a spring gun (with a spring constant of 120 
N/m) to launch a marble vertically into the air. The mass of the 
marble is kg and the spring is compressed 0.04 m.
a)How high will the marble go?
b)How fast will it be going when it leaves the gun?
v=9.8m/s B)
h = 4.897m A)
answer

65. A roller coaster has a velocity of 25 m/s at the bottom of the 
first hill. How high was the hill?
h=31.9m
answer

66. A student uses the lab apparatus shown above
A student uses the lab apparatus shown above. A 5 kg block 
compresses a spring 6 cm. The spring constant is 300 N/m. 
What will the block's velocity be when released?
answer

67. A 5 kg rock is dropped a distance of 1 m onto a spring
A 5 kg rock is dropped a distance of 1 m onto a spring. It 
compresses the spring 2 cm. What is the spring constant?
   k=245000N/
m
answer

68. How much work is done in stopping a 5 kg bowling ball 
rolling with velocity of 10 m/s?
answer

69. How much work is done compressing a spring with a 450 
N/m spring constant 2 cm?
answer