SIMPLE MACHINES
Chapter Nine: Simple Machines
9.1 Types of Simple Machines
9.2 Mechanical Advantage
9.3 Levers in the Human Body
Chapter 9.1 Learning Goals
Define machine.
Identify examples of simple
machines.
Explain the meaning of input and
output.
Investigation 9A
Levers
Key Question:
How does a lever work?
9.1 Using Machines
A machine is a device
with moving parts
that work together to
accomplish a task.
A bicycle is a good
example.
9.1 Using Machines
The input includes everything you do to
make the machine accomplish a task, like
pushing on the bicycle pedals.
The output is what the machine does for
you, like going fast or climbing a steep hill.
9.1 Forces in Machines
A simple machine is an unpowered
mechanical device, such as a lever.
9.1 Review of work
 Every process that is done by machines can
be simplified in terms of work:
1. work input: the work or energy supplied to
the process (or machine).
2. work output: the work or energy that comes
out of the process (or machine).
9.1 Review of energy
A rope and pulley
machine illustrates a
rule that is true for all
processes that
transform energy.
The total energy or
work output can never
be greater than the total
energy or work input.
Chapter Nine: Simple Machines
9.1 Types of Simple Machines
9.2 Mechanical Advantage
9.3 Levers in the Human Body
Chapter 9.2 Learning Goals
Define mechanical advantage in terms
of input and output forces.
Classify levers as first, second, or third
class.
Evaluate the mechanical advantage of
different simple machines.
9.2 Mechanical advantage
Machines multiply
forces.
One person could lift
an elephant—quite a
heavy load—with a
properly designed
system of ropes and
pulleys!
9.2 Mechanical advantage
Mechanical advantage is the ratio of
output force to input force.
mechanical
advantage
MA = Fo
Fi
Input force (N)
Output force (N)
9.2 The Lever
A lever includes a stiff structure (the
lever) that rotates around a fixed point
called the fulcrum.
9.2 The Lever
Levers are useful because you can
arrange the fulcrum and the input arm
and output arm to adjust the
mechanical advantage of the lever.
9.2 The Lever
Each class of levers is defined by the
location of the input and output forces
relative to the fulcrum.
9.2 Gears
Many machines
require that rotating
motion be transmitted
from one place to
another.
Gears change force
and speed.
9.2 Designing Gear Machines
The gear ratio is the ratio of output turns
to input turns.
You can predict how force and speed are
affected when gears turn by knowing the
number of teeth for each gear.
Turns of output gear
Turns of input gear
To = Ni
Ti
No
Number of teeth
on input gear
Number of teeth
on input gear
9.2 Tension
If
the rope
is not moving,
its tension
is
Ropes
and strings
carry tension
forces
along tothe
their length.
equal
force pulling on each end.
9.2 Ramps
A ramp is a simple machine that allows
you to raise a heavy object with less force
than you would need to lift it straight up.
The mechanical
advantage of a
ramp is the ramp
length divided by
the height of the
ramp.
9.2 Screws
A screw is a
rotating ramp.
You find the
mechanical
advantage of a
screw by dividing
its circumference
by the lead.
9.2 Screws
A wedge is like a ramp that
can work while in motion (a
ramp is always stationary).
A wedge has a side that
slopes down to a thin edge.
The mechanical advantage
for a wedge is inversely
related to the size of the
wedge angle.
9.2 Wheel and axle
A wheel rotates around a
rod called an axle.
The mechanical advantage
is the ratio of the radius of
the wheel to the radius of
the axle.
The wheel and axle move
together to move or lift
loads.
Solving Problems
A crowbar is a
type of lever that
you use to pull a
nail out of a piece
of wood.
If the handle of a crowbar is 40
centimeters and the foot is 2
centimeters, what is its mechanical
advantage?
Solving Problems
1. Looking for:
 …mechanical advantage of lever
2. Given
 …input arm = 40 cm; output arm = 2 cm
3. Relationships:
 M.A. = Length of input arm
Length of output arm
4. Solution
 M.A. = 40 cm ÷ 2 cm = 20
Chapter Nine: Simple Machines
9.1 Types of Simple Machines
9.2 Mechanical Advantage
9.3 Levers in the Human Body
Chapter 9.3 Learning Goals
Locate human body parts that act
as levers.
Classify human body as first,
second, or third class levers.
Investigation 9B
Levers and the Human Body
Key Question:
What types of levers does your body have?
9.3 Levers in the human body
The human body is
a complex machine
that includes a
number of simple
machines—levers.
Your arms and legs,
for example, work
as levers to move
and lift objects.
9.3 Levers in the human body
A classic example of a
third class lever is a
broom.
A broom does not
multiply force, but it
does multiply speed.
Since your limbs are
third class levers, they
multiply speed to do
tasks quickly.
9.3 Levers in the human body
Human arms and legs
are examples of third
class levers because
the input forces are
between a fulcrum and
the output force.
The output force is
what you accomplish
with your hands and
feet.
Where is the input
force and the fulcrum?
9.3 Levers in the human body
In the human body, all bones act as
levers and each joint can serve as a
fulcrum.
 When lifting your head, your neck works as
a first-class lever.
 When you stand on your toes, the feet act
as second-class levers.
 When biting, your jaw works as a thirdclass lever.
Investigation 9C
Mechanical Advantage
Key Question:
 What is mechanical
advantage, and how do
ropes and pulleys give
you mechanical
advantage?
Prosthetics in Action
Hugh Herr, Ph.D., a
physicist and engineer
studies biomechanics and
prosthetic technology. In
addition to holding
several patents in this
field, he has developed
highly specialized feet
for rock climbing that are
small and thin.