1. Work, Power, and Machines
9.1

2. Work
A quantity that measures the effects of a force acting over a distance
Work = force x distance
W = Fd

3. Work
Work is measured in:
Nm
Joules (J)

4. Work Example
A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do?

5. Work Example Work = Fd Work = (5200 N)(25m) Work = 130000 N  m
= J

7. Power A quantity that measures the rate at which work is done
Power = work/time
P = W/t

8. Power
Watts (W) is the SI unit for power
1 W = 1 J/s

9. Power Example
While rowing in a race, John uses 19.8 N to travel meters in 60.0 s. What is his power output in Watts?

10. Power Example Work = Fd Power = W/t Power = 3960 J/60.0 s
Work = 19.8 N x m= 3960 J
Power = W/t
Power = 3960 J/60.0 s
Power = 66.0 W

11. Machines
Help us do work by redistributing the force that we put into them
They do not change the amount of work

12. Change the direction of an input force (ex car jack)
Machines
Change the direction of an input force (ex car jack)

13. Machines
Increase an output force by changing the distance over which the force is applied
(ex ramp)
Multiplying forces

14. Mechanical Advantage
A quantity that measures how much a machine multiples force or distance.

15. Mechanical Advantage Input distance Mech. Adv = Output Distance
Output Force
Mech. Adv. =
Input Force

16. Mech. Adv. example
Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.

17. Mech. Adv. Example Input = 6.0 m Output = 1.5 m Mech. Adv.=6.0m/1.5m

18. Simple Machines
9.2

19. Simple Machines Most basic machines Made up of two families Levers
Inclined planes

20. The Lever Family
All levers have a rigid arm that turns around a point called the fulcrum.

21. The Lever Family Levers are divided into three classes
Classes depend on the location of the fulcrum and the input/output forces.

23. First Class Levers Have fulcrum in middle of arm.
The input/output forces act on opposite ends
Ex. Hammer, Pliers

24. First Class Levers
Input Force
Output Force
Fulcrum

26. Second Class Levers Fulcrum is at one end.
Input force is applied to the other end.
Ex. Wheel barrow, hinged doors, nutcracker

27. Second Class Levers
Output Force
Fulcrum
Input Force

29. Third Class Levers
Multiply distance rather than force.
Ex. Human forearm

30. Third Class Levers
The muscle contracts a short distance to move the hand a large distance

31. Third Class Levers
Output distance
Input Force
Fulcrum

34. Pulleys Act like a modified member of the first-class lever family
Used to lift objects

35. Pulleys
Output
Force
Input force

37. The Inclined Plane
Incline planes multiply and redirect force by changing the distance
Ex loading ramp

38. The Inclined Plane
Turns a small input force into a large output force by spreading the work out over a large distance

40. Functions like two inclined planes back to back
A Wedge
Functions like two inclined planes back to back

41. A Wedge
Turns a single downward force into two forces directed out to the sides
Ex. An axe , nail

43. Or Wedge Antilles
from Star Wars

44. Not to be mistaken
with a wedgIEEEEE

45. Inclined plane wrapped around a cylinder
A Screw
Inclined plane wrapped around a cylinder

46. A Screw
Tightening a screw requires less input force over a greater distance
Ex. Jar lids

48. Compound Machines A machine that combines two or more simple machines
Ex. Scissors, bike gears, car jacks

51. Energy

52. Energy and Work Energy is the ability to do work
whenever work is done, energy is transformed or transferred to another system.

54. Energy Energy is measured in: Joules (J)
Energy can only be observed when work is being done on an object

55. Potential Energy PE
the stored energy resulting from the relative positions of objects in a system

56. Potential Energy PE
PE of any stretched elastic material is called Elastic PE
ex. a rubber band, bungee cord, clock spring

59. Gravitational PE
energy that could potentially do work on an object do to the forces of gravity.

60. Gravitational PE
depends both on the mass of the object and the distance between them (height)

61. Gravitational PE Equation
grav. PE= mass x gravity x height
PE = mgh or
PE = wh

62. PE Example
A 65 kg rock climber ascends a cliff. What is the climber’s gravitational PE at a point 35 m above the base of the cliff?

63. PE Example PE = mgh PE=(65kg)(9.8m/s2)(35m) PE = 2.2 x 104 J
PE = J

64. Kinetic Energy the energy of a moving object due to its motion.
depends on an objects mass and speed.

65. Kinetic Energy What influences energy more: speed or mass?
ex. Car crashes
Speed does

66. Kinetic Energy Equation
KE=1/2 x mass x speed squared
KE = ½ mv2

67. KE Example
What is the kinetic energy of a 44 kg cheetah running at 31 m/s?

68. KE Example
KE = ½ mv2
KE= ½(44kg)(31m/s)2
KE=2.1 x 104 J
KE = J

70. Mechanical Energy
the sum of the KE and the PE of large-scale objects in a system
work being done

71. Nonmechanical Energy
Energy that lies at the level of atoms and does not affect motion on a large scale.

72. Atoms Atoms have KE, because they at constantly in motion.
KE  particles heat up
KE  particles cool down

73. Chemical Reactions
during reactions stored energy (called chemical energy)is released
So PE is converted to KE

74. Other Forms
nuclear fusion
nuclear fission
Electricity
Light

75. Energy Transformations
9.4

77. Conservation of Energy
Energy is neither created nor destroyed
Energy is transferred

78. Energy Transformation
PE becomes KE
car going down a hill on a roller coaster

79. Energy Transformation
KE can become PE
car going up a hill KE starts converting to PE

81. Physics of roller coasters