There are three types of levers,
identified as first class, second class,
and third class. The difference is
where the fulcrum and applied force
are, in relation to the load. Each
class works differently, and is used
to do different jobs
• The first class lever uses a
fulcrum in between, and the
applied force and load are at
opposite ends.
The diagram at the left shows a
first class lever set up to move a
heavy load with a small applied
force. The force must be applied
over a long distance, in order to
make the heavy load move just a
small amount.
By adjusting how far the fulcrum
is from the load, you can control
the mechanical advantage. The
closer it is to the load, the more
force is applied.
• Eg scissors
• The second class lever is one
where the fulcrum is at one
end, and the applied force at
the other. The load that is to
be moved is between them.
This lever is different in how it
works ... it causes the load to
move in the same direction as
the force you apply.
Just as with a first class lever,
how close the load is to the
fulcrum determines by how
much your force will be
multiplied. If you want to
move a very large load with a
small applied force, you must
put the load very close to the
fulcrum.
• Eg. Wheel barrow
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The fulcrum is once again at one end
of the lever, but this time the load is
at the other end, and you apply a
force in between.
This lever can not give any
mechanical advantage. Regardless of
where you apply the force, the force
you apply must always be greater
than the force of a load.
If you were using this lever to lift an
object at a distance, it would require
less force to just stand above it and
lift it up ... using the lever will require
more force!
So why use a third class lever at all?
The answer lies in the fact that the
load moves in the same direction as
the force you apply, which is
convenient. So is the application of
force between the load and the
fulcrum
Eg. Your arm
http://www.worsleyschool.net/science/files/lever/page2.html
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When a device reduces the effort
needed to lift a load, there is an
advantage.
Levers and pulleys are examples of
simple machines.
Two single pulleys can be used to
construct a multiple pulley system.
Pulley systems can be described using
diagrams.
There is a relationship between the
number of ropes supporting a load in
a pulley system and the effort
required to lift a load.
There is a cost associated with the
advantage in a pulley system.
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A pulley is an object that is usually
round with a smooth groove around
its outside edge.
A pulley transfers a force along a rope
without changing its magnitude.
assume that the rope through the
groove of a pulley moves smoothly
and evenly, without catching. They
say it moves without friction.
When two rough surfaces are rubbed
together (like two wooden blocks),
they become warm; the heat is
caused by friction. If the two surfaces
were slicked with oil and then rubbed
together, they would move much
more smoothly and very little heat
would be generated. There is much
less friction.
also assume that the pulley and rope
weigh very little compared to the
weight on the end of the rope, so
they can ignore these two weights
and make their calculations with only
the heavy weight on the end of the
rope.
There is a force (tension) on the rope that is equal to the weight of the object. This
force or tension is the same all along the rope. In order for the weight and pulley (the
system) to remain in equilibrium, the person holding the end of the rope must pull
down with a force that is equal in magnitude to the tension in the rope. For this
simple pulley system, the force is equal to the weight, as shown in the picture. The
mechanical advantage of this system is 1! The output force is the weight to be held in
equilibrium and the input force is the applied force.
In the second figure, the pulley is moveable. As the rope is pulled up, it can also move
up. The weight is attached to this moveable pulley. Now the weight is supported by
both the rope end attached to the upper bar and the end held by the person! Each
side of the rope is supporting the weight, so each side carries only half the weight (2
upward tensions are equal and opposite to the downward weight, so each tension is
equal to 1/2 the weight). So the force needed to hold up the pulley in this example is
1/2 the weight! Now the mechanical advantage of this system is 2; it is the weight
(output force) divided by 1/2 the weight (input force).
The mechanical advantage of each system is easy to determine. Count the number of
rope segments on each side of the pulleys, including the free end. If the free end is to
be pulled down, subtract 1 from this number. This number is the mechanical
advantage of the system! To compute the amount of force necessary to hold the
weight in equilibrium, divide the weight by the mechanical advantage! In the third
figure, for example, there are 3 sections of rope. Since the applied force is downward,
we subtract 1 for a mechanical advantage of 2. It will take a force equal to 1/2 the
weight to hold the weight steady. The fourth figure has the same two pulleys, but the
rope is applied differently and it is pulled upwards. The mechanical advantage is 3, and
the force to hold the weight in equilibrium is 1/3 the weight. Each additional figure
shows another possible pulley configuration and lists the force necessary to lift and
hold the weight still. The mechanical advantage for the system will be the number in
the denominator of the force.
• These systems are known as simple pulley systems
because they use the same rope throughout the
system. If the pulleys were attached with several
different ropes (not one continuous rope), the system
would be a complex pulley system. The force necessary
to hold a complex pulley system in equilibrium would
have to be computed using other Statics methods.
Once it was known, however, the mechanical
advantage of the system would still be computed by
dividing the weight to be held by the force applied to
hold it!
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http://www.swe.org/iac/lp/pulley_03.html