1. WORK AND MACHINES
CHAPTER 8

2. LESSON 4 PART 1: USING
LEVERS
 A screwdriver, used to pry the lid from a can, is an
example of a simple machine
 A simple machine is a tool with few parts that
makes it easier or possible to do work
 Simple machines change the direction,
distance, or size of the force you apply

3. THE LEVER
 A lever is a simple machine containing a bar
that can turn around a fixed point
 The fixed point of a lever is called a fulcrum
 Levers can have many shapes

4.  In the figure, a person is using a lever to move a
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boulder; as the person pushes down, the boulder
moves up
The force the person applies to the machine is called
effort force
The object to be lifted, the boulder, is called the
resistance.
The force the machine uses to move the resistance is
called the resistance force
The force the machine exerts is greater than the
force the person exerts, SO using a machine makes

5. THE THREE CLASSES OF
LEVERS
 Levers can be grouped into three classes
 The classes of levers are based on the position
of the resistance force, the fulcrum, and the
effort force
 In a first-class lever, the fulcrum is positioned
between the effort and the resistance
 Using a screwdriver to open a paint can is an
example of a first-class lever

6.  In a second-class lever, the resistance is
positioned between the effort and the fulcrum
 Wheelbarrows, paper cutters, and most
nutcrackers are examples of second-class levers

7.  In a third-class lever, the effort is positioned
between the fulcrum and the resistance
 A broom is an example of a third-class lever

8. http://www.youtube.com/watch?v=SprRX6mSsEg&
safety_mode=true&persist_safety_mode=1

9. LESSON 4 PART 2: USING
LEVERS
 Energy cannot be created or destroyed; and,
because energy is the ability to do work, work
cannot be created either
 No simple machine can do more work than the
person using it supplies
 Machines can increase or change the direction
of the force a person exerts; and, some machines
allow a person to use less force to do the same
amount of work

10. SCIENCE MYTH
 Myth:
 A machine is something that has been
manufactured
 Fact:
 Our bodies are machines. They contain all three
classes of levers. When you lift your head forward
or back, you use a first-class lever. When you
stand on your toes, you use a second-class lever.
When you hold a weight in your hand with your arm
extended, you are using a third-class lever.

11.  The amount of work a person puts into a machine
is called the work input
 Work input equals the person’s effort force
multiplied by the distance of that effort

work input = fe × de
 The amount of work actually done by the machine
against the resistance is called the work output
 Work output equals the resistance force
multiplied by the distance the resistance moved

work output = fr × dr

12.  Work output can never be greater than work
input because energy cannot be created
 The efficiency of a machine measures how
much useful work it can do compared with how
much work was put into it

efficiency = work output/work input × 100%
 Efficiency is written as a percent, and multiplying
by 100 tells you what percent of the work input is
converted to work output
 ALL machines have efficiencies that are less
than 100 percent

13. PRACTICE PROBLEM
 Suppose a woman uses a lever to lift a crate. She
applies 120 newtons of effort force, and she pushes
her end of the lever 1.0 meters. The machine exerts
400 newtons of resistance force, and it lifts the
crate 0.2 meters. What is the work input, the work
output, and the efficiency of the lever?
 work input = effort force × effort distance

work input = 120 newtons × 1.0 meters = 120 joules
 work output = resistance force × resistance distance

work output = 400 newtons × 0.2 meters = 80 joules
 efficiency = work output/work input × 100%

efficiency = 80/120 × 100% = 66.7%