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CHAPTER 13
Class
Date
Work and Energy
SECTION
2 Simple Machines
KEY IDEAS
As you read this section, keep these questions in mind:
•
•
•
•
What are simple machines?
What simple machines are in the lever family?
What simple machines are in the inclined plane family?
What are compound machines?
What Are Simple Machines?
We are surrounded by many different electronics and
machines. In physics, a machine is a mechanical device
that changes the motion of an object. Remember that
machines make work easier by changing the way a force
is applied. Many machines, such as cars and bicycles,
are complicated. However, even the most complicated
machine is made from a combination of just six simple
machines. Simple machines are the most basic machines.
Scientists divide the six simple machines into two families: the lever family and the inclined plane family. The
lever family includes the simple lever, the pulley, and the
wheel and axle. The inclined plane family includes the
simple inclined plane, the wedge, and the screw.
READING TOOLBOX
Compare As you read
this section, make a chart
showing the similarities and
differences between the six
simple machines. Describe
how each machine affects
input and output forces
and distances. Include the
mechanical advantage each
machine provides.
The lever
family
Simple lever
Pulley
Wheel and axle
The
inclined
plane
family
Simple inclined
plane
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1. Infer What do you think
is the reason that the wedge
and the simple inclined
plane are in the same family
of simple machines?
Screw
Wedge
How Do Levers Work?
If you have ever used a claw hammer to remove a nail
from a piece of wood, you have used a simple lever. All
levers have a rigid arm that pivots, or turns, around a
point. This point is called the fulcrum.
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Simple Machines continued
CLASSES OF LEVERS
Scientists divide levers into three main classes based
on where the fulcrum, input force, and output force are.
In a first-class lever, the fulcrum is between the input
and output forces. The mechanical advantage of a firstclass lever depends on the position of the fulcrum.
First-class lever
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2. Identify What part of the
hammer acts as the fulcrum
when the hammer is used to
remove a nail?
Output
force
Input
force
Fulcrum
Fulcrum
In a first-class lever, the fulcrum is between the input force and the output force.
In a second-class lever, the output force is located
between the fulcrum and the input force. Wheelbarrows
are examples of second-class levers. The mechanical
advantage of a second-class lever is always greater than
one. Therefore, second-class levers multiply force.
Second-class lever
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Output
force
3. Compare How do the
directions of the input and
output forces in a secondclass lever compare?
Input
Input
force
force
Fulcrum
Fulcrum
In a second-class lever, the output force is between the fulcrum and the
input force.
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4. Apply Concepts How
does the input distance of a
third-class lever compare to
the output distance?
In a third-class lever, the input force is located
between the fulcrum and the output force. The output
force of a third-class lever is less than the input force.
Therefore, the mechanical advantage of a third-class lever
is less than one.
Third-class levers multiply distance. A person’s forearm is a third-class lever. The elbow is the fulcrum. The
biceps muscle attaches to the bone near the elbow. The
muscle contracts a short distance to move the hand a
long distance.
Third-class lever
Output
force
Fulcrum
Fulcrum
Input
Input
force
In a third-class lever, the input force is between the fulcrum and the output force.
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Simple Machines continued
PULLEYS
A pulley is another kind of simple machine in the lever
family. You may have used a pulley to lift things, such as
a flag on a flagpole.
As shown below, the point in the middle of a fixed
pulley is like the fulcrum of a lever. The rest of the pulley acts like the fixed arm of a first-class lever, because
it pivots around a point. The distance from the fulcrum
is the same on both sides of a fixed pulley. A fixed pulley
has a mechanical advantage of one. Therefore, a fixed
pulley does not increase force. It simply changes the
direction of the force.
A moveable pulley or a combination of pulleys can
produce a mechanical advantage greater than one.
Moveable pulleys are attached to the object being moved.
Fixed and moveable pulleys can be combined into a
single unit to produce a greater mechanical advantage.
Input
force =
150 N
Input
force =
75 N
Output
force =
150 N
MA = 1
A fixed pulley has a
mechanical advantage of
one. It changes only the
direction of a force.
Output
force =
150 N
Input
force =
50 N
Output
force =
150 N
MA = 2
A single moveable
pulley has a mechanical
advantage of two.
MA = 3
Combining several
pulleys produces an
even higher mechanical
advantage.
READING CHECK
5. Describe How does a
fixed pulley change the input
force?
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6. Infer Of the three
pulleys in the figure, which
one requires the largest
input distance? Explain your
answer.
WHEEL AND AXLE
A wheel and axle is made of a lever or pulley (wheel)
connected to a shaft (axle). Bicycle gears, doorknobs,
wrenches, and screwdrivers are wheel-and-axle machines.
When a small input force turns a wheel, the output force
is multiplied. The output force increases because the
axle is smaller than the wheel. Therefore, the axle rotates
through a smaller distance. The figure at the top of the
next page shows a wheel and axle.
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Simple Machines continued
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Output
force
7. Identify What are the
input and output forces on a
steering wheel?
Input
force
Fulcrum
A wheel and
axle changes
a small input
force into a
larger output
force.
What Are Inclined Planes?
READING CHECK
8. Explain How do ramps
reduce the force needed to
move an object?
Remember that a ramp makes work easier by reducing
the force needed to move an object. Ramps are examples
of simple inclined planes. Simple inclined planes are
simple machines with a straight, slanted surface.
Inclined planes, like all machines, do not reduce the
amount of work needed to move an object. They reduce
the force you apply by increasing the distance over which
you apply it. You do the same amount of work whether
you lift something straight up or push it up a ramp.
The mechanical advantage of an inclined plane is equal
to its length (input distance) divided by its height (output distance). Pushing an object up a long, gradual ramp
takes less force than pushing the object up a short, steep
ramp. In other words, a long, gradual inclined plane has a
greater mechanical advantage than a short, steep plane.
Output
force
Input
force
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9. Apply Concepts A
student wants to use one of
the two ramps in the figure
to move a box of books. If
the student wants to use the
smallest possible force to
move the books, which ramp
should he use? Explain your
answer.
A short, steep inclined
plane has a small
mechanical advantage.
Output
force
Input
force
A long, gently sloping
inclined plane has a
greater mechanical
advantage.
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Simple Machines continued
WEDGES AND SCREWS
A wedge is formed from two inclined planes placed
back to back. Ax blades, splitting wedges, and doorstops are examples of wedges. As shown in the figure
below, using a wedge is like pushing down on a ramp.
In other words, a wedge is a moving inclined plane. A
wedge turns a single downward force into two forces
directed out to the sides. It both multiplies force and
changes the direction of the force.
Input
force
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10. Describe How does the
wedge affect the input force?
Output
force
A wedge is a moving inclined plane.
A screw is an inclined plane wrapped around a cylinder.
When you tighten a screw with gently sloping threads, you
apply a small input force over a large distance. The screw
then exerts a large output force over a small distance. A
screw with steeper threads requires more force to tighten,
because the input force is applied over a shorter distance.
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11. Identify Relationships
How is a screw related to a
simple inclined plane?
A screw is an inclined plane wrapped around a cylinder.
What Are Compound Machines?
Many of the things you use every day are compound
machines. A compound machine is a machine that combines two or more simple machines. For example, a pair
of scissors is made of two first-class levers joined at a
common fulcrum. Each lever arm is a wedge. Together, the
wedges can cut paper. Bicycles and cars are made of many
simple machines, so they are also compound machines.
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Section 2 Review
SECTION VOCABULARY
compound machine a machine made of more
than one simple machine
simple machine one of the six basic types of
machines, which are the basis for all other
forms of machines
1. List What are the six simple machines?
2. Compare How does the input distance of a single fixed pulley compare to the out-
put distance? Explain your answer. (Hint: How are force and distance related?)
3. Infer Why are wedges and screws part of the inclined plane family of simple
machines?
4. Compare How are second-class levers and third-class levers similar? How are
they different?
5. Explain How are fixed pulleys similar to first-class levers?
6. Infer Can a simple inclined plane have a mechanical advantage less than one?
Explain your answer. (Hint: Can the height of a ramp ever be greater than its length?)
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