1) A lever is a simple machine that uses a bar to change the direction or magnitude of a force. It has three main parts: the fulcrum (point of support), the effort (input force), and the resistance (output force).
2) There are three classes of levers depending on the position of the fulcrum relative to the effort and resistance.
3) Levers allow a smaller input force to produce a larger output force, but the total work output can never exceed the total work input due to the law of conservation of energy. Machines like levers can increase force but reduce the distance it acts through.
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
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. 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
9. 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.
10. 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
11. 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
12. 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%
