TEKS
Name _________________________ Class ______ Date _________
Grade 6
Lesson 6.8E
TEKS 6.8E Investigate how inclined planes and pulleys can be used to
change the amount of force to move an object.
TEKS Lesson 6.8E: Machines
What is a machine?
A machine is a device that allows you to do work in a way that is easier. You may think
of machines as complex gadgets with motors, but a machine can be quite simple. For
example, a shovel makes the work of moving soil easier, so a shovel is a machine.
You use the same amount of work to move a pile of soil whether you use your hands or a
shovel. The shovel, or any other machine, changes the way in which work is done. A
machine makes work easier by changing at least one of three factors. A machine may:
· change the amount of force you exert,
· change the distance over which you exert a force, or
· change the direction in which you exert a force.
1. Identify What is the advantage of using a machine to do work?
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
Input and Output Forces When you do work, you exert a force over some distance.
Although a machine changes how you do the work, you still exert a force. For example,
you exert a force on the shovel when you use it to lift soil. This force is called the input
force. The input force moves the machine a certain distance, called the input distance.
The machine, in turn, does work by exerting a force on an object. This is the output
force. The output force is exerted over a distance called the output distance.
Input and Output Work The input force multiplied by the input distance represents an
amount of work called the input work. The output force times the output distance is the
output work. When you use a machine, the amount of output work can never be greater
than the amount of input work.
1
Name _________________________ Class ______ Date _________
TEKS
Grade 6
Lesson 6.8E
Changing Force For some machines, the output force is greater than
work:
Work = Force Ő Distance
If the amount of work does not change, a decrease in force requires an increase in
distance. So, for a machine to do work with less input force, you must apply that input
force over a greater distance.
Suppose you want to lift a heavy box onto a stage. Lifting the box requires a lot of force
because you are doing work on the box by moving it against the force of gravity. Instead
of lifting the box, you could push it up a ramp. The length of the ramp is greater than the
height of the stage, so you exert your input force over a greater distance. However, when
you use the ramp, the work is easier because you can exert a smaller input force and
receive a greater output force.
2. Explain How can a machine exert a force that is greater than the force
you put into it?
______________________________________________________________________
______________________________________________________________________
2
Name _________________________ Class ______ Date _________
Changing Distance For some machines, the output force is less than
the input force. How does this type of machine help you do work?
TEKS
Grade 6
Lesson 6.8E
are holding the chopsticks a short distance. As a result, the other ends of
the chopsticks move a greater distance, allowing you to pick up food.
The food end moves a greater distance, but the force is less. Another example is when
you ride a bicycle. In high gear, you apply a force to the pedals over a short distance. The
bicycle, meanwhile, travels a much longer distance.
Changing Direction Some machines don't change force or distance. Instead, they
change the direction of the force. For example, a steel cable runs over a small wheel at
the top of the weight machine in the figure below. You can raise the weights by pulling
down on the cable. The force is the same but this cable system is a machine that makes
your job easier by changing the direction in which you exert your force.
3
Name _________________________ Class ______ Date _________
How does an inclined plane change force?
TEKS
Grade 6
Lesson 6.8E
As you read above, a ramp makes it much easier to lift something, such
as pushing a grocery cart over a curb. A ramp is an example of a simple
machine called an inclined plane. An inclined plane is a flat, sloped
surface.
An inclined plane allows you to exert your input force over a longer distance, as shown
below, so the input force needed is less than the output force. The input force that you use
on an inclined plane is the force with which you push or pull an object. The output force
is the force that you would need to lift the object without the inclined plane. This force is
equal to the weight of the object.
3. Apply Give an example of an inclined plane.
______________________________________________________________________
Mechanical Advantage The mechanical advantage of a machine is the ratio by
which it reduces the force needed to do work. You can determine the mechanical
advantage of an inclined plane by dividing the length of the incline by its height.
Mechanical advantage =
Length of incline
Height of incline
For example, if you are loading a truck that is 1 meter high using a ramp that is 3 meters
long, the mechanical advantage of the ramp is 3 meters ĝ 1 meter, or 3. The inclined
plane increases the input force to three times as much. If the height of the incline does not
change, increasing the length of the incline will increase the mechanical advantage. The
longer the incline, the less input force you need to push or pull an object.
4
Name _________________________ Class ______ Date _________
TEKS
Grade 6
Lesson 6.8E
How does a pulley change force?
When you raise a flag on a flagpole or when you open and close window
blinds, you are using a pulley. A pulley is a simple machine made of a
grooved wheel with a rope or cable wrapped around it, as shown below. You use a pulley
by pulling on one end of the rope. The force of your pull is the input force. At the other
end of the rope, the output force pulls up on the object you want to move. To move a
heavy object over a distance, a pulley can make work easier in two ways:
· The pulley can change the direction of your input force.
· The pulley can decrease the amount of input force needed to lift the object.
Types of Pulleys A fixed pulley has a single wheel that does not move. It changes the
direction of the force but not the amount of force. For example, you pull down on the
flagpole rope, and the flag moves up. Moveable pulleys and systems of pulleys, such as
the block and tackle shown in the figure below, decrease the amount of force needed to
lift an object. As the number of sections of rope that support the object increases, the
input force decreases.
The mechanical advantage of a pulley is equal to the number of sections of rope that
support the object. As the force decreases, the distance increases. So, if the mechanical
advantage is 2, you pull 2 meters of rope to lift the object 1 meter.
5
Name _________________________ Class ______ Date _________
TEKS
Grade 6
Lesson 6.8E
Lesson Check
1. Define What is a machine?
___________________________________________________________________
___________________________________________________________________
2. Identify Which of the following factors is not changed by a machine?
A force
B work
C distance
D direction
3. Identify A gardener uses a shovel, which is a simple machine, to dig a hole in the
soil for planting a tree. Identify which part of the shovel represents the:
a. input force
__________________________________________________________________
b. output force
__________________________________________________________________
c. input distance
__________________________________________________________________
d. output distance
__________________________________________________________________
4. Calculate What is the mechanical advantage of a 2.0 m inclined plane that rises to
a height of 0.5 m?
6
Name _________________________ Class ______ Date _________
TEKS
Grade 6
Lesson 6.8E
___________________________________________________________________
5. Infer If the mechanical advantage of a machine is less than 1, what
can you infer about the machine?
A The input force is less than the output force.
B The input distance is less than the output distance.
C The input work is less than the output work.
D The input distance is equal to the output distance.
6. Compare How is the effect on direction of force of a single fixed pulley like a single
moveable pulley?
___________________________________________________________________
___________________________________________________________________________
7. Analyze A ramp is used to lift a 250-kg piano onto a stage 1 m high. How does the
ramp affect the:
a. force that the piano mover must apply?
__________________________________________________________________
__________________________________________________________________
b. distance over which the piano has to travel?
__________________________________________________________________
___________________________________________________________________
7