N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
7th Grade Science
Unit 2
Simple Machines
Spring 2006
SIMPLE MACHINES
In this 7th grade unit, the student will demonstrate basic relationships between
force and motion using simple machines. Students should be become familiar with
all six simple machines: pulley, lever, inclined plane, wedge, wheel and axle, screw.
This is the first time that simple machines will have been introduced. It is not
covered in any grade before 7th grade, nor is it covered in 8th grade. Students will
analyze the effect of changing force or distance in simple machines, and
demonstrate mechanical advantage of various machines. It is more important for
students to have a conceptual understanding of mechanical advantage than to be
able to calculate it.
Machines and Work
A simple machine is a device that makes work easier by changing the size or
direction of the force applied to it. Students should be introduced to the concept
of work because it is an integral part of understanding simple machines. Work is
done when a force causes an object to move in the direction the force is applied.
The mathematical equation for calculating work is force times distance (W = f × d).
This equation is present on the TAKS formula chart, so students need to have
practice utilizing it. They do not, however, need to be able to manipulate the
formula to solve for force or distance. Teachers can give students examples of
when work is or is not done. For example, no work is in done in reading a book, but
work is done is turning the pages. Emphasize that you work is done only when a
force is used to move an object and the motion must be in the direction of the
force.
Machines come in many shapes and forms, but there are only six types of simple
machines. These simple machines, however, can be combined to form complex
(compound) machines. Work in everyday life is often done with the aid of
machines. The use of a machine requires two types of work: work input and work
output. Work input is the work effort supplied by the person. Work output is the
work done by the machine. Students should be familiar with the different terms
used when referring to input and output force. Input force can also be referred to
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
1
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
7th Grade Science
Unit 2
Simple Machines
Spring 2006
as effort and output force can also be called the resistance or load. It should be
understood that machines do not change the amount of work done. They just make
it easier by either changing the size or direction of the force needed to do the
work.
The Lever
A lever is a simple machine consisting of a bar that pivots around a fixed point
called a fulcrum. There are three types of levers based on the location of the
fulcrum, load and input force or effort. It is more important for students to be
able to conceptually understand the different classes of levers than to be able to
classify them as 1st, 2nd, or 3rd class. The three classes of levers are discussed
here to aid teacher understanding.
In a first-class lever the fulcrum is between the input force and the output force,
or load. The first class lever always changes the direction of the input force, as
with a see-saw. When the input force is applied downward, then the load moves up.
Depending on the location of the fulcrum, the first class lever can be used to
increase or decrease the force needed to lift the load. Teachers can illustrate this
concept by the following activity: Place one pencil on a flat surface and another
pencil on top of it, forming a cross. Lay a book on one end of the top pencil. Push
down on the free end of the top pencil to raise the book. Change the position of
the bottom pencil, moving it closer then farther from the book. Observe and
record how changing the position of the pencil affects the force needed to raise
the book.
1st class lever:
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
2
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
7th Grade Science
Unit 2
Simple Machines
Spring 2006
In a second-class lever, the load or output force is between the fulcrum and the
input force. A second class lever allows the user to apply less force than the force
exerted by the load. Because the output force is greater than the input force, the
input force must be exerted over a greater distance.
2nd class lever:
In a third –class lever, the input force is between the fulcrum and the load. Third
class levers are helpful because they increase the distance through which the
output force is exerted. The output force and input force move in the same
direction as when you swing a baseball bat. In this example, the ball moves in the
same direction as you swing the bat. The fulcrum would be the hand, the force is
the bat and the load or output force is the ball.
3rd class lever:
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
3
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
7th Grade Science
Unit 2
Simple Machines
Spring 2006
The following table provides examples of simple machines that are levers:
Simple Machine
Lever Class
see-saw
1
hammer’s claws
1
pliers
1
(2 sets)
scissors
1
(2 sets)
bottle opener
2
wheelbarrow
2
door opening
(w/ hinges)
2
baseball bat
3
fishing rod
3
tweezers
3
(2 sets)
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
4
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
7th Grade Science
Unit 2
Simple Machines
Spring 2006
The Pulley
A pulley consists of a grooved wheel that holds a rope or cable. The load is
attached to one end of the rope, and the input force is applied to the other end of
the rope. The number of rope segments determine how easy it is to lift the load.
The more rope segments, the easier it is to lift the load. The mechanical advantage
of a pulley can be determined by the number of rope segments supporting the load.
A common mistake students make is to count the handle as a rope segment when it
does not support the load. Examples of the use of pulleys in everyday include
flagpoles, blinds, wells, and cranes.
A pulley that is attached to something to hold it steady is called a fixed pulley. A
fixed pulley makes work easier by changing the direction of the force. A fixed
pulley lets you to take advantage of the downward pull of your weight to move a
load upward. The distance the rope is pulled is equal to the distance the object is
lifted.
Fixed pulley:
In a moveable pulley system, one end of the rope is attached but the wheel is free
to move, hence a moveable pulley. The load is attached to the moveable wheel. A
movable pulley does not change the direction in which the force moves, it multiplies
the force and increases the distance the force moves. Since the load is supported
by two sections of rope, only half the force is needed for a fixed pulley.
Movable pulley:
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
5
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
7th Grade Science
Unit 2
Simple Machines
Spring 2006
When a fixed pulley and movable pulley are used together, a pulley system is
created. Pulley systems may have several wheels. This system can be called a
compound pulley or block and tackle and has the highest mechanical advantage.
Pulley Systems:
The Inclined Plane
An inclined plane has a flat surface raised on one end. It is essentially a ramp. A
long ramp has a greater mechanical advantage and is easier to push something up
than a short ramp. Even though a person using the long ramp must travel a farther
distance, less force is needed. A ramp used to get a person in a wheelchair into a
building is a good example of an inclined plane.
force
Inclined Plane
The Wedge
A wedge is formed when two inclined planes join together to make a sharp edge.
Wedges are used to split or lift objects. Axes, knives, nail points, doorstops and
plows are all examples of wedges. Many organisms also use wedges as tools. A
woodpecker’s beak and the root of a seed are examples. The following picture of a
seed’s root acting as a wedge appeared in the August 2005 TAKS information
booklet:
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
6
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
7th Grade Science
Unit 2
Simple Machines
Spring 2006
The Wheel and Axle
A wheel and axle is a simple machine that consists of a shaft, called the axle,
inserted through the middle of a wheel. Both the wheel and the axle are two
circular objects. The axle is the smaller of the two circular objects that are
moving. When a force is applied to the wheel, it is magnified when it is
transferred to the axle. This makes the output force larger than the input force
because the larger diameter wheel has to be turned less than the smaller diameter
axle. Screwdrivers, pencil sharpeners, bicycle wheels, and faucet handles, and
doorknobs are all examples of a wheel and axle. Gears are an example of a
modified wheel and axle in which the teeth can be classified as wedges.
Gears
The Screw
A screw is an inclined plane that is wrapped around a cylinder. The ridges around
the shaft of the screw are called threads. As the screw is turned, the threads
pull the object up the shaft. The greater the number of threads and the longer
the shaft, the easier it is to turn the screw. This equates to a greater mechanical
advantage as well. Jar lids, bottle caps, and simple nail-like screws are all examples
of this simple machine.
Source: www.nvsd44.bc.ca
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
7