1. Table of Contents
What Is Work?
How Machines Do Work
Simple Machines

2. The Meaning of Work - What Is Work?
Work is done on an object when the object moves in the same direction in which the force is exerted.
To do work on an object:
The object must move when force is applied.
The object must move in the same direction as the force.

3. Calculating Work and Power
- What Is Work?
Calculating Work and Power
Force is measured in Newtons (N).
Formula for Work:
Work = Force x Distance
The SI unit of measure for work is joule (J), in honor of James Prescott Joule, who studied work in the mid-1800s.
Formula for Power:
Power = Work OR Force x Distance Time Time
The SI unit of measure for power is Watts (W), in honor of James Watt who greatly improved the steam engine.

4. Calculating Power - What Is Work?
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?
Read and Understand
What information have you been given?
Force of the tow truck (F) = 11,000 N
Distance (d) = 5.0 m
Time (t) = 25 s

5. Calculating Power - What Is Work?
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?
Plan and Solve
What quantity are you trying to calculate?
The Power (P) the tow truck uses to pull the car = __
What formula contains the given quantities and the unknown quantity?
Power = (Force X Distance)/Time
Perform the calculation.
Power = (11,000 N X 5.0 m)/25 s
Power = (55,000 N•m)/25 s or 55,000 J/25 s
Power = 2,200 J/s = 2,200 W

6. Calculating Power Practice Problem - What Is Work?
A motor exerts a force of 12,000 N to lift an elevator m in 6.0 seconds. What is the power produced by the motor?
16,000 W or 16 kW

7. Asking Questions - What Is Work?
Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions.
Question
Answer
What is work?
Work is done when an object moves in the same direction in which the force is exerted.
How can you calculate work?
Work = Force X Distance
What is power?
Power is the rate at which work is done.

8. Input and Output Work - How Machines Do Work
The amount of input work done by the gardener equals the amount of output work done by the shovel.
Input force – effort you put into the machine.
Output force – effort the machine puts into an object.

9. What Is a Machine? - How Machines Do Work
A machine makes work easier by changing at least one of three factors:
The amount of force you exert
The distance over which you exert your force
The direction in which you exert your force.
Ex. A ramp or faucet
Ex. Hockey stick, chopsticks, riding a bike
Ex. A weight machine with pulleys

10. - How Machines Do Work
Efficiency
Efficiency determines how much work was wasted due to friction.
Efficiency = Output Work x 100%
Input Work
The higher the percentage, the more efficient the machine (wastes little work).
An ideal machine would have 100% efficiency.

11. Mechanical Advantage - How Machines Do Work
Mechanical advantage = Output force
Input force
The input force and output force for three different ramps are shown in the graph.

12. Mechanical Advantage - How Machines Do Work Reading Graphs:
What variable is plotted on the horizontal axis?
Input force

13. Mechanical Advantage - How Machines Do Work Interpreting Data:
If an 80-N input force is exerted on Ramp 2, what is the output force?
400 N

14. Identifying Main Ideas
- How Machines Do Work
Identifying Main Ideas
As you read the section “What Is a Machine?” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.
Main Idea
The mechanical advantage of a machine helps by…
Detail
Detail
Detail
changing the amount of force you exert
changing the distance over which you exert your force
changing the direction of the force

15. Links on Mechanical Efficiency
- How Machines Do Work
Links on Mechanical Efficiency
Click the SciLinks button for links on mechanical efficiency.

16. End of Section: How Machines Do Work

17. Inclined Plane - Simple Machines
An inclined plane is a flat, sloped surface.
Ideal mechanical advantage = Length of incline
Height of incline

18. Wedge - Simple Machines
A wedge is a device that is thick at one end and tapers to a thin edge at the other end.
Ideal Mechanical Advantage = Length of Wedge
Width of Wedge

19. Screws - Simple Machines
A screw can be thought of as an inclined plane wrapped around a cylinder.
I.M.A. = Length around the threads
Length of the screw

20. Levers - Simple Machines
A lever is a ridged bar that is free to pivot, or rotate, on a fixed point (fulcrum).
I.M.A. = Distance from fulcrum to input force
Distance from fulcrum to output force

21. Levers - Simple Machines
Levers are classified according to the location of the fulcrum relative to the input and output forces.

22. Wheel and Axle - Simple Machines
A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.
I.M.A. = Radius of wheel
Radius of axle

23. Pulley - Simple Machines
A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.
I.M.A. = Number of sections of rope that support the object

24. Simple Machines in the Body
Most of the machines in your body are levers that consist of bones and muscles. Your teeth are wedges.

25. Compound Machines - Simple Machines
A compound machine is a machine that utilizes two or more simple machines.
I.M.A. = The product of the individual I.M.A.s of the simple machines that make it up.

26. Three Classes of Levers
- Simple Machines
Previewing Visuals
Before you read, preview Figure 17. Then write two questions that you have about the diagram in a graphic organizer like the one below. As you read, answer your questions.
Three Classes of Levers
Q. What are the three classes of levers?
A. The three classes of levers are first-class levers, second-class levers, and third-class levers.
Q. How do the three classes of levers differ?
A. They differ in the position of the fulcrum, input force, and output force.

27. Click the Video button to watch a movie about levers.
- Simple Machines
Levers
Click the Video button to watch a movie about levers.

28. Click the Video button to watch a movie about pulleys.
- Simple Machines
Pulleys
Click the Video button to watch a movie about pulleys.

29. End of Section: Simple Machines

30. Graphic Organizer Simple Machine Mechanical Advantage Example
Inclined plane
Length of incline ÷ Height of incline
Ramp
Wedge
Length of wedge ÷ Width of wedge Ax
Length around threads ÷ Length of screw
Screw
Screw
Distance from fulcrum to input force ÷ Distance from fulcrum to output force
Lever
Seesaw
Wheel and axle
Radius of wheel ÷ Radius of axle
Screwdriver
Pulley
Number of sections of supporting rope
Flagpole

31. End of Section: Graphic Organizer