Section 4
Simple Machines
Key Concepts
• What are the six types of simple
machines?
• What determines the mechanical
advantage of the six types of simple
machines?
• The six types of simple machines are
the lever, the wheel and axle, the
inclined plane, the wedge, the screw,
and the pulley.
Weekly
Invention
LaborSaving
Auto Jack
The idea for this labor-saving auto jack
comes from Rube Goldberg (1883–1970),
a sculptor, author, and Pulitzer Prizewinning cartoonist.
Levers
"Give me a place to stand on, and I can move the earth."
The Works of Archimedes with the Method of Archimedes,
Levers
• a lever, a rigid bar that is free to move
around a fixed point.
• The fixed point the bar rotates around is
the fulcrum.
• Levers are classified into three categories
based on the locations of the input force,
the output force, and the fulcrum.
Levers
• The input arm of a lever is the distance
between the input force and the fulcrum.
• The output arm is the distance between
the output force and the fulcrum.
• There are three classes of Levers: 1st
class, 2nd class, and 3rd class
Mechanical Advantage
of a Lever
• To calculate the ideal mechanical
advantage of any lever, divide the input
arm by the output arm.
Output
50 N
Input
10 N
MA = 50N/10N
MA = 5
First-Class Levers
• The position of the fulcrum identifies a
first-class lever
• the fulcrum of a first-class lever is always
located between the input force and the
output force.
Input
Output
force
Input force
Output Force
Fulcrum
force
First-Class Levers
• Depending on the location of the fulcrum,
the mechanical advantage of a first-class
lever can be greater than 1, equal to 1, or
less than 1.
• Examples of first-class levers include a
seesaw, scissors, and tongs.
Fear of Physics
Second-Class Levers
• In a second-class lever the output force is
located between the input force and the
Input
Output
fulcrum.
force
force
Fulcrum
Third-Class Lever
• The input force of a third-class lever is
located between the fulcrum and the
output force.
Fulcrum
Input
force
Output
force
Input
force
Fulcrum
Output
force
Wheel and Axle
• A wheel and axle is a simple machine
that consists of two disks or cylinders,
each one with a different radius.
• The wheel is the larger object and the axle
is the smaller object.
Input
Output
Steering Shaft
(Axle)
Steering Wheel
(Wheel)
Output
Input
Screwdriver
Shaft (Axle)
Screwdriver
Handle (Wheel)
Mechanical Advantage
of a Wheel and Axle
• To calculate the ideal mechanical
advantage of the wheel and axle, divide
the radius (or diameter) where the input
force is exerted by the radius (or
diameter) where the output force is
exerted.
Input
Output
MA = WR/AR
MA = 10cm/2cm
MA = 5
Pulleys
• A pulley is a simple machine that consists
of a rope that fits into a groove in a wheel.
• Pulleys produce an output force that is
different in size, direction, or both, from
that of the input force.
A worker watches as
a pulley moves a large
fabricated
part through a factory.
Three Types of Pulleys
• There are 3 types of pulleys: Fixed Pulley,
Moveable Pulley, and Block and Tackle
Fixed Pulley
• A fixed pulley is a
wheel attached in a
fixed location.
• Fixed pulleys are only
able to rotate in place.
• The direction of the
exerted force is
changed by a fixed
pulley, but the size of
the force is not
•MA = 1
Movable Pulley
• A movable pulley is
attached to the object
being moved rather than
to a fixed location.
• The size of the force is
changed not the dirction.
• If you are pulling up on
the rope on the right with
a force of 10 newtons,
then both sections of the
rope pull up with the
same force of 10
newtons.
2 ropes pulling on
the object
MA = 2
Pulley System
(Block and Tackle)
• By combining fixed
and movable pulleys
into a pulley system,
a large mechanical
advantage can be
achieved.
• The mechanical
advantage that results
depends on how the
pulleys are arranged.
2 ropes pulling on
the object
MA = 2
Mechanical Advantage of a Pulley
• The ideal mechanical advantage of a
pulley or pulley system is equal to the
number of rope sections supporting the
load being lifted.
MA = 2
MA = 5
Inclined Planes
• An inclined plane is a slanted surface
along which a force moves an object to a
different elevation.
• No moving parts
• The less slanted the inclined plane, the
longer the distance over which the force is
exerted and the more the force is
multiplied
Mechanical Advantage of an
Inclined Plane
• The ideal mechanical advantage of an
inclined plane is the distance along the
inclined plane divided by its change in
height.
• Formula:
• Distance of inclined plane / height of
inclined plane
3m
6m
1m
1m
Wedge
• A wedge is a V-shaped object whose
sides are two inclined planes sloped
toward each other.
• An inclined plane that moves
• Most wedges are made
Input
of two inclined planes
Output
Mechanical Advantage of a Wedge
• The longer and thinner a wedge, the
greater the mechanical advantage
• How do you make a knife cut better?
• Sharpen it (make it longer and thinner)
Screw
• A Screw is an inclined plane wrapped
around a cylinder.
• Multiplies force by acting through a long
distance
• The closer the threads (pitch),
the longer the distance
Mechanical Advantage of a Screw
• The mechanical advantage of a screw
increases when the threads are close
together
1 mm
2 mm
Compound Machines
• A compound machine is a combination
of two or more simple machines that
operate together.
The complex workings
of this watch consist of a series of
machines. The output of one
machine acts as the driving input
for the next machine in the series.
Reviewing Concepts
• 1. Name six kinds of simple machines. Give an example
of each.
• 2. Describe how to determine the ideal mechanical
advantage of each type of simple machine.
• 3. How are the lever and the wheel and axle related to
each other?
• 4. What is the ideal mechanical advantage of a ramp if
its length is 4.0 m and its higher end is 0.5 m above its
lower end?
• 5. Tightening a screw with a larger spacing between its
threads requires fewer turns of a screwdriver than
tightening a screw with smaller thread spacing. What is a
disadvantage of the screw with larger thread spacing?
• 6. What class or classes of lever always have a
mechanical advantage greater than 1?