1. Do Work!!!

2. What is work??

3. What are some examples of work that you do?

4. What are some examples of work that you do?

5. Work
The transfer of energy that occurs when a force makes an object move
So if you push against something and it doesn’t move, have you done work?
No Way!!!

6. Conditions for work to be done
The applied force must make the object move
The movement must be in the same direction as the applied force.

7. Example
Is picking up books from the floor work?
Yes it is!

8. Example
When you carry books while walking, are you doing work?
No way! Why?

9. Work and Energy It’s a family thing!

10. Work and Energy are related
Whenever work is done, energy is ALWAYS transferred

11. Think about picking up a heavy stack of books Remember, when an object’s height above Earth’s surface increases, its GPE increases
You transferred energy from your muscles to the books and increased its GPE by increasing its height!

12. Lets Get
Mathematical

13. Work Equation
Work = Force(distance) or W= F(d) *SI units: Work = Joules Force= Newtons Distance= meters

14. Example
You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m, how much work do you do?

15. Example
A lawn mower is pushed with a force of 80 N. If 12,000 J of work are done in mowing a lawn, what is the total distance the lawn mower was pushed?

16. Do you have the power?

17. You and a friend decide to have a race to see who can move a box the fastest. You each have boxes that weigh the same and you push them the same distance. However, your friend reaches the finish line faster than you! What is the same in this example? What is different in this example?

18. In the previous example your friend has more power than you do!

19. What is power?
The amount of work done in one second

20. Equation
Power = Work Time or *SI Units: P = W Power= Watts t Work= Joules Time= Seconds

21. Example
You do 900 J of work in pushing a sofa. If it took 5 s to move the sofa, what was your power?

22. Example
A light bulb produces 100 W of power. How much work does the light bulb do in 5 seconds?

23. Potential Energy Stored energy due to position
Ex: Apple hanging in a tree

24. Types of Potential Energy
Elastic Potential Energy- energy stored by something that can stretch or compress
- Ex: rubber band or spring
Chemical Potential Energy- energy stored in chemical bonds
- Ex: Food and gasoline

25. Gravitational Potential Energy
Energy stored by objects due to their position above Earth’s surface
Depends on the object’s mass and height above the ground

26. Gravitational Potential Energy
Equation
GPE = mgh
SI unit: Joule; J

27. Example
What is the gravitational potential energy of a ceiling fan that has a mass of 7 kg and is 4 m above the ground?

28. Elastic Potential Energy
Energy available when a deformed elastic object returns to its original position
E.P.E. = ½(k)(x^2)
k: spring constant a.k.a. the numerical value representing the “stretchiness” of an elastic object.

29. Example
When a 2 kg mass is attached to a vertical spring the spring is stretched from 0.1 m to 0.5 m. If the spring constant is N/m find the elastic potential energy of the spring.

30. Equation Power equation for energy transfer Power = energy transferred
time Or
P= E t

31. Chapter 5 Work and Machine
Section 2

32. Bellringer
To lift a baby from a crib 100 J of work are done. How much power is needed if the baby is lifted in 0.5 seconds?

33. What is a machine??
Name some
Examples

34. Using Machines Machine- device that makes doing work easier Examples:
Knives, scissors, and doorknobs!

35. How do machines make work easier?
1. Increase Force
Example: A car jack (This increases the applied force)

36. How do machines make work easier?
2. Increasing the distance over which a force is applied
Example: Think of a moving truck
Is it easier to use a ramp or pick up a couch and set it in the truck?

37. How do machines make work easier?
3. Changing the direction of an applied force
Example:
An axe
Pulley
Car Jack

38. Two types of forces involved when machines do work
1. Input force- The force that is applied to the machine
Symbol: Fin
2. Output force- force applied by the machine
Symbol: Fout

39. Two types of work done when using a machine
1. Input work- work done by you on a machine
Symbol: Win
2. Output work- work done by the machine
Symbol: Wout

40. Is energy always conserved?

41. Since energy is neither created nor destroyed, the amount of energy transferred by machine to the object is not greater than the amount of energy you exerted on the machine.
*Wout is never greater than Win

42. HEAT Not all of a machine’s energy is transferred to the object.
Some of it changes to what?
HEAT

43. Wout is always smaller than Win
Therefore,
Wout is always smaller than Win

44. Ideal Machine
In an ideal machine there is no friction.
Win = Wout

45. Mechanical Advantage The ratio of the output force to the input force.
Equation:
Mechanical Advantage= output force (newtons)
input force (newtons) Or
MA = Fout
Fin

46. Example
Calculate the mechanical advantage of a screwdriver if the input force is 25 N and the output force is 3,000 N.

47. Efficiency
Measure of how much of the work put into a machine is changed into useful output work by the machine.
Equation:
Efficiency = Wout x %
Win

48. Example
Find the efficiency of a machine that does 800 j of work if the input work is 2400 J.

49. Chapter 5 Work and Machines
Section 3 Simple Machines

50. Simple Machine
A machine that does work with only one movement of the machine
Six Types:
Lever
Pulley
Wheel and Axle
Inclined plane
Screw
Wedge

51. Levers A bar that is free to pivot or turn around a fixed point
Fixed point is called the fulcrum
Examples:
Wheelbarrow
Rake
Baseball Bat

52. First-Class Lever
The fulcrum is located between the input and output forces
Example:
Screwdriver used to open a paint can
*Draw Figure 13 p.138

53. Second-Class Lever
The output force is located between the input force and the fulcrum.
Example:
Wheelbarrow
*Draw Figure 13 p. 138

54. Third-Class Lever
The input force is applied between the output force and the fulcrum.
Example:
Baseball Bat
*Draw Figure 13 p.138

55. Ideal Mechanical Advantage of a Lever
Equation:
IMA = Lin (length of input arm)
Lout (length of output arm)

56. Pulley
A grooved wheel with a rope, chain, or cable running along the groove
A fixed pulley is a modified first-class lever

57. Wheel and Axle
A simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axle rotate togther.
Example:
Doorknobs
Pencil Sharpener

58. Mechanical Advantage of the Wheel and Axle
Equation:
IMA = radius of wheel (m)
radius of axle (m)

59. Inclined Planes
A sloping surface that reduces the amount of force required to do work
Example:
Ramp

60. Mechanical Advantage of an Inclined Plane
IMA = length of slope (m)
height of slope (m)

61. The Screw Inclined Plane wrapped in a spiral around a cylindrical post
Input force – when you turn the screw
Output force- exerted along
the threads of the screw

62. The Wedge Inclined plane with one or two sloping sides Example: Knife
Axe

63. Compound Machines Two or more simple machines that operate together
Example:
Can opener
Car