1. Chapter 12 Work and Machines
8th Grade

2. Section 1- What is Work? What example of work can you think of? Work
Lifting your backpack
Taking your books out of your backpack
Pushing a child on a swing
Work
When you exert a force on an
object that causes the object to
move some distance.
Work is done on an object the object MOVES in the same direction in which the force is exerted.

3. 2 Requirements for Work:
1) motion
2) distance
There is no work without motion!
Ex. You may exert a force on a car stuck in the snow but unless the force you exert makes the car move then you have done no work.

4. What does direction have to do with work?
Lifting books out of a backpack is work, but carrying them to class is not.
Why didn’t you do any work when you carried your books to school??
Answer: To do work on an object, the force you exert must be in the same DIRECTION as the objects MOTION.

5. How is Work Calculated??
The amount of work done on an object can be determined by multiplying force times distance.
Work = Force X Distance
Joules (J)
The amount of work you
do when you exert a force of 1
Newton to move an object a
distance of 1 meter.
Let’s work a few examples….. 

6. Calculating Work Examples
How much work does it take to lift a 50 newton potted plant 0.5 meters off the ground onto a table?
Work = Force X Distance
Work = 50 Newton X 0.5 Meters
Work = 25 J

7. Calculating Work Examples
How much work does it take to lift a 100 newton plant onto a table that is 0.5 meters off the ground?
Work = Force X Distance
Work = 100 X 0.5
Work = 50 J

8. Calculating Work Examples
Is more work done when a force of 2 N moves an object 3m or when a force of 3N moves an object 2m?
2N X 3 = 6J
3N X 2 = 6J
The work done in the two situations is equal!

9. Power The rate at which work is done.
Power equals the amount of work done on an object in a unit of time.
Factor that affects power?
Time
Ex. You need more power to
run up the stairs when your
backpack than to walk because
it takes you less TIME.
Ex. Applying more power to an object means doing more work in the same time.

10. Calculating Power Power = Work / Time Units:
(J/s) Joules per second or Watts
(W)
What if you aren’t given work?
Power = (Force X Distance) / Time

11. Calculating Power Examples
A small motor does 5000J of work in 30 seconds. What is the power of the motor in watts?
Power = Work / Time
P= 5000 J / 30 s
P = Watts

12. Calculating Power Examples
A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck?
Power = (Force X Distance) / Time
Power = (11,000 N X 5.0 m) / 25 s
Power = 2,200 Watts or 2,200 J/s

13. Work and Power Practice Problems- Handout
Copy Questions and Work out Solutions on your own paper.

14. Section 2- How Machines Do Work
A device that allows you to do work in a way that is easier or more effective.
Changes the amount of force you exert or the distance over which you exert the force.
Ex. Shovel- makes moving soil easier
Machines make work EASIER by changing at least 1 of 3 factors:
1) amount of force you exert
2) distance over which you exert the force
3) direction in which you exert the force

15. Input and Output Forces
When you use a machine, you exert a force.
Ex. You exert a force on the shovel when you use it to lift soil
Input Force
Force applied to a machine.
Ex. Exerting a force on the shovel moves it a certain distance.
Output force
The force the machine exerts on an object.
Ex. Shovel exerts a force over another distance

16. Input and Output Work Refer to page 413 Input Work Output Work
Input force X input distance
The gardener exerts a large input force over a small input distance
Output Work
Output force X output distance
The shovel exerts a small output force over a large output distance.

17. Mechanical Advantage
A machines mechanical advantage is the number of times a machine increases a force exerted on it.
Compares the input and output FORCE of a machine.
Formula:
Mechanical Advantage = (Output) / Input

18. Practice Problems
You exert an input force of 10 newtons on a hand-held can opener, and the opener exerts an output force of 30 newtons on a can. What is the mechanical advantage of the can opener?
Output Force / Input Force
30 N / 10 N = 3
*Increasing Force

19. Practice Problems
Suppose your input force is 20 newtons and the machine’s output force is 10 newtons. What is the machine mechanical advantage?
Output force / Input force
10 N / 20 N = 0.5
* Increasing Distance
If only the direction change, the input force will be the same as the output force. The mechanical advantage = 1.

20. Practice Problems
If a machines has an input force of 40N and an output force of 80N, what is its mechanical advantage?
Output force/ Input force
80N / 40 N
= 2

21. Refer to handout given during class for more examples of mechanical advantage problems.

22. Efficiency of Machines
***IDEAL SITUATION ***
Work you put into a machine is EXACTLY equal to the work done by the machine.
In REAL situations the output work is ALWAYS less than the input work.
WHY???
In every machine, some work is wasted overcoming the force of FRICTION!
Without friction there would be equal input and output work
An ideal machine would have an efficiency of 100%.
All machines have an efficiency < 100%

23. Efficiency
Compares output and input WORK of a machine expressed as a percentage.
Formula:
Efficiency = (Output work / Input work) X 100%
Let’s Practice!

24. Efficiency Calculations
You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower?
(Output work / Input work) X 100%
(200,000 / 250,000) X 100%
The efficiency of the lawn mower is 80%

25. Efficiency Calculations
The input work you do on a can opener is 12 J. The output work the can opener does is 6 J. What is the efficiency of the can opener?
(Output work/ Input work) X 100%
(6 J) / 12J = .5 J X 100%
= 50%

26. Section 3 – Simple Machines
6 Types of Simple Machines:
1) Inclined Plane
2) Wedge
3) Screw
4) Lever
5) Wheel and Axle
6) Pulley

27. Inclined Plane Simple Machine Flat, sloped surface
Allows you to exert your input force over a longer distance.
Ex. A ramp in a parking garage
Lengthening the ramp will INCREASE its ideal mechanical advantage.

28. Wedge
Simple Machine
Thick at one end and tapers to a thin edge at the other end.
Thought of as an inclined plane that moves.
Ex. As you bite into a peach, your front teeth act as a wedge.
Incisors are shaped like wedges!

29. Screw
Simple Machine
Can be thought of as an inclined plane wrapped around a cylinder.

30. Levers A rigid bar that is free to pivot, or rotate, on a fixed point.
The fixed point that the lever rotates around is called the fulcrum.
Most of the machines in your body consist of bones and muscles are levers.
Ex. Waving your hand at the wrist.
Joints act as the fulcrums of levers in your body.
Ex. When you raise your leg, your hip acts as a fulcrum for the upper leg.
Ex. Using a paint opener to open a can of paint

31. 3 Classes of Levers
Pg. 427
A second class lever always multiplies force.
A hockey puck is considered a machine because it multiplies distance.

32. Wheel and Axle Simple Machine
Made of two circular or cylindrical objects fastened together that rotate about a common axis.
Ex. Screwdriver

33. Pulley
A simple machine made of a grooved wheel with a rope or cable wrapped around it.
Ex. Raise and lowering a flagpole

34. Compound Machines
Utilizes two or more simple machines
Ex. Bicycles