1. Work and Simple Machines Chapter 14
I can :
1. Recognize when work is done
2. Calculate how much work is done
3. Explain the relation between work and
power.
It took the ancient Egyptians more than 100 years to build the pyramids without modern machines. But now even tall skyscrapers can be built in a few years. Complex or simple, machines have the same purpose they make doing a job easier.

2. What is work?
In Science,
Work is done when a force causes an object to move in the same direction that the force is applied.

3. In order for you to do work: Two things must occur:
You must apply force on an object.
(Force is a push or pull that changes the motion or shape of an object.)
2.The object must move in the same direction as your applied force.
Demonstrate with cut out arrows

4. You do work on an object ONLY when the object moves as a result of the force you exerted.
Work involves MOTION, not just effort.

5. Explain why the woman does no work on the bags of groceries if she is standing still.
A force must move an object in the same direction that the force is applied.
The woman may think she is working by holding the bags. However, if she is not moving, she is NOT doing work because she is not causing something to move.

6. To do work, how must a force make an
Object move?

7. In the direction of the force.
When the boy
walks forward
NO WORK is being done
by the upward force that his
arms exert.

8. Work is done only when the FORCE you exert on an
object is in the same direction as the object’s motion;
Lifting a clothes basket is work, but carrying it
while walking is not work.

9. The boys arms DO work when they exert an
Upward force on the weights and the weights move
Upward.
Whenever you apply force to an object,
the object moves in the direction of the
force.

10. Applying a force ALWAYS results in work being done.
True or False:
Applying a force ALWAYS results in work
being done.
False

11. Force in two directions:
Sometimes only part of the force you exert moves an object.
Think about what happens when you push a lawn mower.
You push at an angle to the ground.
Only the part of the
Force that is in the SAME
direction as the motion of
the mower does work.

12. Calculating Work
Work is done when a force makes an object move.
More work is done when the force is increased or the object is moved a greater distance.
In SI units, the unit for work is the joule, named for James Prescott Joule
Work Equation
Work ( joules) = force ( newtons ) x distance( meters)
W=Fd

13. Distance in the work equation is the distance
an object moves only WHILE the force is
applied.

14. Applying Math
Work - A painter lifts a can of paint that weights 40 N a distance
Of 2 m. How much work does she do?
Hint: to lift a can weighing 40 N, the painter must exert a
Force of 40 N.
This is what you know: force: F = 40 N
distance: d = 2m
This is what you need to find out: work: W = ?J

15. This is the procedure you need to use:
Substitute the known values F = 40 N and d = 2 m
Into the work equation:
W =Fd = (40 N) (2m) = 80 Nm = 80 J
Check your answer by dividing the work you
Calculated by the distance given in the problem.
The result should be the force given in the problem.
Do questions on page 428

16. Practice Problems:
As you push a lawn mower, the horizontal force is 300 N. If you push the mower a distance of 500 m, how much work do you do?
A librarian lifts a box of books that weighs 93 N a distance of 1.5 m. How much work does he do?

17. Answers:
Work = force x distance = 300 N x 500 m = 150,000J
Work = force x distance = 93N x 1.5 m = 140 J

18. What is Power?
Imagine two weightlifters lifting the same amount
of weight the same vertical distance. They both do
the same amount of work. However, the amount
of power they use depends on how long it took to do
The work.
Power is the rate at which work is done.(How quickly work
Is done.)
The weightlifter who lifted the weight in less time is
more powerful.

19. In SI units, the unit of power is WATT, in honor
Of James Watt, a nineteenth-century British
Scientist who invented a practical version of the steam engine.

20. Calculating Power:
Power can be calculated by dividing the amount of WORK DONE by the TIME NEEDED to do the work.
Power Equation
power = work (joules)
time (seconds)
P = W T
Page 429

21. Applying Math
Power - You do 200 J of work in 12 s. How much power
Did you use?
Work : W = 200J ; time: t = 12 s
Power: P = ? Watts
P=W =200 J = 17 watts
T s
Check your answer by multiplying the power you
Calculated by the time given in the problem.
The results should be the work given in the problem.

22. Practice Problems
In the course of a short race, a car does 50,000 J of
work in 7 s . What is the power of the car during the race?
2. A teacher does 140 J of work in 20 s. How much power did
Use?

23. Answers:
Power = work done/time needed
50,000 J/7s = 7,143 watts
2. P =W/t = 140 J/20s = 7 watts

24. What is Energy? The ability to do work.
Example: I must have energy to run the mile.

25. Work and Energy
If you push a chair and make it move, you do work
on the chair and change its energy. Recall that when something is moving it has energy of motion, or kinetic energy. By making the chair move you increase its
kinetic energy.
You also change the energy of an object when you do work and lift it higher. An object has potential energy that increases when it is higher above Earth’s surface.
By lifting an object, you do work and increase its
potential energy.

26. Power and Energy:
When you do work on an object you INCREASE the KINETIC energy of the object. Because energy can never be created or destroyed, if the object gains energy then you must lose energy.
When you do work on an object you transfer energy to the object, and your energy decreases. The amount of work done is the amount of ENERGY TRANSFERRED in a certain amount of time.
Sometimes energy is transferred even when no work is done, such as when heat flows from a warm to a cold object. In fact, there are many ways energy can be transferred even if no work is done. Power is always THE RATE at which energy is transferred- the amount of energy transferred divided by the time needed.

27. Is carrying a book while walking work? Why or why not?
No because the books motion is not in the same direction as the force applied to it.

28. Today we have learned:
1. Work is done when a force causes an object
to move in the same direction that the force is applied.
2. Power is the rate at which work is done.
3. Something is more powerful if it can do a given
amount of work in less time.

29. 1. When a force causes motion to occur in the
Review Questions:
1. When a force causes motion to occur in the
Same direction in which the force has been applied, we say that _____________has been done.
2. Suppose you are waiting for a train. While
You are standing on the platform, your arms are becoming more and more tired from holding heavy
Suitcases. Are you doing work?
1. Work is done when an object moves in the same direction a force is applied.2. No, when you lifted the suitcases you were doing work because you applied a force and moved an object. Holding the bags, although tiring, isn’t considered wprk.

30. In SI, the unit for work is the ______________. Ampere Joule Newton
Watt
The unit for work is the joule

31. I can:
Explain how a machine makes work easier
Calculate the mechanical advantages and efficiency
of a machine.
Explain how friction reduces efficiency.

32. What is a machine?
A machine is simply a device that makes doing work easier.

33. What is a Simple Machine?
A simple machine has few or no moving parts.
Simple machines make WORK EASIER.
Even a sloping surface can be a machine.
Every day people do jobs that require something to be moved. To move an object a force must be applied to the objects. Machines make moving objects easier by changing the force that must be applied to do the job. Work is done when an object is moved. However, a machine doesn’t reduce the amount of work needed, In fact the amount of work done usually is greater when a machine is used due to the effects of friction.

34. Mechanical Advantage
Machines do not decrease the amount of work you need to do.
A machine changes the WAY in which you do work.
You exert a force over some distance.

35. Input Force The force you apply on a machine is the INPUT FORCE.
The work you do on the machine is equal to the input force times the distance over which your force moves the machine.
The work that you do on the machine is the input work.
The work you do on the machine is the input force

36. Output Force
The machine also does work by exerting a force to move an object over some distance.
The resistance force.
When using a machine, the output work can never be GREATER than the input work.

37. When you use a machine, the output work can
Never be greater than the input work.

38. Changing Force Some machines make doing work easier by
reducing the force you have to apply to do work.
This type of machine increases the input force, so that the output force is greater than the input force.

39. Changing Force
The number of times a machine increases the input force is the mechanical advantage of the machine.

40. Mechanical Advantage What is the advantage of using a machine?
A machine makes work easier by changing the amount of force you need to exert, the distance over the force exerted, or the direction in which you exert your force.

41. Mechanical Advantage
Number of times the input force is multiplied by a machine-Equation:
MA=F out/F in
Calculate the mechanical advantage of a car jack if 100N on the handle of the jack and the jack applies a force of 2500N to the car. MA=25 MA=2500N/100N

42. Efficiency
The ability of a machine to convert input work to output work; calculated as :
efficiency=output work/input work x 100%
eff = W out x 100
W in

43. Friction
To help understand friction, imagine pushing a heavy box up a ramp.
As the box begins to move, the bottom surface of the box slides across the top of the ramp.
Neither surface is perfectly smooth-each has high spots and low spots.
Go to other power point section 2 on friction

44. Friction Reduces efficiency by converting some work into HEAT.
The efficiency of a real machine is always LESS THAN 100 percent because of friction.
OIL or another lubricant, can increase efficiency by reducing the number of contact points between surfaces. Lubrication can reduce the friction between two surfaces. Two surfaces can stick together where the surfaces come in contact. Adding oil or another lubricant separates the surfaces so that fewer high spots make contact.

45. Friction and Efficiency
One way to reduce friction between two surfaces is to add oil.
Oil fills the gaps between surfaces and keeps many of the high spots from making contact.
More input work is converted to output work by machine.

47. Infer how the output work done
by a machine changes when friction is
reduced.

48. Discussion Question:
What is the efficiency of a machine and how can it be improved?
Efficiency is the ability of a machine to convert input work to output work. It can be improved by using lubricant to reduce friction.

49. We have learned there are three main advantages to a machine.
By changing the amount of force you need to exert.
2 .Changing the distance over which the force is
exerted
3. Changing the direction in which you exert the force.

50. Exit slip:
What is the force that you apply on a machine?
The force that the machine applies is the_________.

51. Simple Machines
I can statements:
Distinguish among the different simple machines
Describe how to find the mechanical advantage of each machine.

52. Simple machine does work with only ONE movement;
a machine made of a COMBINATIOM of simple machines
is a COMPOUND machine.
The six simple machines are :
1. Incline plane
2. lever
3. Wheel and axle
4. Screw
5. Wedge
6. Pulley
Examples of a compound machine: can opener
and bicycle

53. Inclined Plane
To move limestone blocks weighing more than 1,000 kg each, archaeologists hypothesize that the Egyptians built enormous ramps. A simple machine known as an incline plane.

54. Inclined Planes
An inclined plane is a flat surface that is higher on one end (sloped surface)
Inclined planes make the work of moving things easier
A sloping surface, such as a ramp. An inclined plane can be used to alter the effort and distance involved in doing work, such as lifting loads. The trade-off is that an object must be moved a longer distance than if it was lifted straight up, but less force is needed. You can use this machine to move an object to a lower or higher place.  Inclined planes make the work of moving things easier.  You would need less energy and force to move objects with an inclined plane. 

55. Less FORCE is needed to move an object from one height to another using an inclined plane than is needed to lift the object.
As the inclined plane gets longer, the force needed to move the object gets SMALLER.
The mechanical advantage of an inclined plane is the length of the inclined plane divided by its height.

56. Wedges Two inclined planes joined back to back.
Wedges are used to split things.
Inclined plane that moves-wedge changes the direction of the applied force.
Example: Your Front teeth
Doorstop

57. Wedges in Your Body
The teeth of meat eaters, or carnivores,
are more wedge shaped than the teeth of plant eaters.
Carnivores use their teeth to cut and rip meat.

58. Screws
A screw is an inclined plane wrapped around a shaft, cylinder or post.
The inclined plane allows the screw to move itself when rotated.

59. The mechanical advantage of the screw is the length of the inclined plane wrapped around the screw divided by the length of the screw.( drill bit, screws)

60. LEVER- any rigid rod or plank
That pivots about a point.
The point about which the lever pivots is FULCRUM
Mechanical advantage-divide the distance from the fulcrum to the INPUT force by the distance from the fulcrum to the OUTPUT force.
Levers can be divided into classes depending on the position of the FULCRUM.

61. Levers-First Class
In a first class lever the fulcrum is in the middle and the load and effort is on either side
Think of a see-saw
Fulcrum is between effort and resistance

62. Levers-Second Class
In a second class lever the fulcrum is at the end, with the load in the middle
Think of a wheelbarrow
Resistance is between effort and fulcrum

63. Levers-Third Class
In a third class lever the fulcrum is again at the end, but the effort is in the middle
Think of a pair of tweezers
Effort is between resistance and fulcrum

64. Lever Notes:
“F R E”
1st Class- Fulcrum is between effort and resistance “ E F R”
2nd Class-Resistance is between effort and fulcrum “ E R F”
3rd Class-Effort is between resistance and fulcrum “R E F “

65. Wheel and axle- Two circular objects of different sizes that rotate together.
Examples: Steering wheel, doorknob, screw driver

66. The advantage of a wheel and axle is found
by dividing the radius of the wheel by the radius of the axle
In some cases, the input force turns the wheel, and the axle exerts the output force, resulting in a mechanical advantage greater than one’ examples are doorknob, a steering wheel and a screwdriver.
In other cases, the input force turns the axle, and the wheel exerts the output force resulting
in a mechanical advantage LESS than one; example Fan and a Ferris wheel

67. Wheels and Axles The wheel and axle are a simple machine
The axle is a rod that goes through the wheel which allows the wheel to turn
Gears are a form of wheels and axles

68. Pulleys Pulley are wheels and axles with a groove around the outside
A pulley needs a rope, chain or belt around the groove to make it do work

69. Two types of Pulleys:
Fixed 2. Moveable
Fixed Pulleys, such as on window blinds or flagpoles, are attached to an overhead structure and change the direction of the force you exert; they have mechanical advantage of One.

70. Moveable pulleys are attached to the object being lifted and allow you to exert a smaller force; they have a mechanical advantage of two
Pulley system:
a combination of fixed and moveable pulleys

71. Simple Machines
Simple Machines can be put together in different ways to make complex machinery

72. What is the difference between a
simple machine and a compound
machine?

73. A simple machine makes only ONE
Movement; a compound machine is
Made up of multiple simple machines
And makes more than one movement.