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A machine is a device that helps people
do work against some resistive force. Some machines are
powered by engines, motors and even animals, while many
others simply use human power. The basis of all complex
machines comes from their simple components. When these
components are by themselves, they are called simple
machines. The most common simple machines are the lever,
rollers, the ramp, and the pulley.
Questions you may have are:
-
What is the background of machines?
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How does a lever, roller, and pulley
work?
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What about machines that run
forever?
Background of inventions
Before engines and motors were invented,
people had to do things like lifting heavy loads by
hand. Using an animal could help, but what they really
needed is some clever ways to either make work easier or
faster. Ancient people invented, simple machines that
would help them overcome resistive forces and allow them
to do the desired work against those forces.
The ancient Egyptians, for example, used
such inventions to help them build the pyramids. They
used levers to pick up large blocks of stone. They put
those blocks on rollers to move from one area to
another. Then they used ramps to move the blocks up to
the top of the pyramid they were building.
The ancient Romans used catapults to
throw stones at their enemies. The catapult was a large
lever. They used a pulley to pull down the arm of the
catapult. The device was set on wheels--an advanced
version of rollers--to move it from place to place.
We still use those simple machines
today, by themselves and as part of more complex
machines.
The lever
In order to lift up a heavy object, such
as a huge rock, the lever was invented. We still use it
today to lift things. It also can be used as a
teeter-totter and a catapult.
Push on E to lift up the weight R with
this lever
Suppose you wanted to lift up a box that
weighed 200 pounds. If you used a lever with the
distance from the pivot point or fulcrum F to the weight
RA = 1 foot and the other distance EA = 10 feet, then
you would only have to push down at E with 20 pounds of
force.
This is because of the rule that E * EA
= R * RA. In other words,
20 pounds times 10 feet = 200 pounds times 1 foot.
Although you have what they call a
mechanical advantage in being able to lift this heavy
weight, you are actually doing the same amount of work,
because you have to push the 20 pound force 10 times as
far as the 200 pound box moves. For example, to life the
box 3 inches, you have to push the 20 pound force for 30
inches.
Since Work is Force times Distance, then
you can see that:
20 pounds x 30 inches = 200 pounds x 3
inches.
A teeter-totter is a lever that children
use as a play thing. Since the children are usually
approximately the same weight, the fulcrum is placed in
the center of the board.
A child sits on each end of the board,
and they take turns lifting each other off the ground.
A catapult is a lever in reverse. A
heavy weight is dropped on the short side of the level,
catapulting a lighter weight from the longer side of the
lever. This works because not only does the weight on
the long arm of the lever (EA in the picture above)
travel a greater distance, but it also goes faster.
If EA = 10 feet, RA = 1 foot, and you
drop R = 200 pounds at 32 feet per second, weight E = 20
pounds will fly in the air at 320 feet per second!
There are different configurations for
levers, where the fulcrum is placed on one end or the
other. The principles still hold.
Rollers
The resistive force of friction is one
of the first thing ancient people wanted to overcome. In
order to construct buildings they had to drag large
blocks from one place to another. Friction made the job
difficult.
One way to reduce friction is to
lubricate the sliding surface with oil. But this was not
practical in most situations. Then someone came up with
the idea of placing logs or rollers under the object, so
that the friction was greatly reduced.
They used a lever to lift the object
onto the rollers, and then used the rollers to reduce
friction and easily move the heavy object from one place
to another.
After hundreds of years, rollers were
replaced by the invention of the wheel on an axle. A
wheel is simply a refinement of the roller.
Movers use a rack with wheels called a
dolly to move heavy furniture. Where else have you seen
wheels being used to reduce the force of friction?
Ramp
Although rollers reduce friction, a ramp
may be used to gain a mechanical advantage or to reduce
the amount of force required. But remember that the
amount of work required is the same (not counting the
losses due to friction).
A ramp is also called an inclined plane.
By rolling an object up a hill or a ramp, you require
less strength than required to pick the object up the
same height, but you compensate by traveling a greater
distance. This ability to move an object to another
height works the same as with a lever.
"Hey, stick-man. Roll that barrel up the
ramp!"
If you lifted a barrel that weighed 200
Newtons up 6 meters in height, the work against gravity
would be 200 x 6 = 1200 Nm. If you rolled that barrel up
a ramp 12 meters long, you would only have to push with
a force of 100 N. That is because 200 x 6 = 100 x 12.
(Note that by rolling instead of
sliding, you are drastically reducing the extra
resistance from friction.)
Variations on the ramp or inclined plane
include the door-jam or wedge and the threads of a screw
or bolt. Threads are simply a ramp that are wrapped
around a cylinder like a bolt.
The pulley
A pulley is a way to use your own weight
to lift an object to another height. The same force must
be used, but it simply changes direction. You pull down
and the weight goes up.
Pull on the rope to lift the heavy
weight
Perpetual motion machines
Many people have tried to invent a
machine that would run forever, once you got it started.
They call such a machine a perpetual motion machine.
Of course, the big problem with a
machine running forever concerns the losses due to
friction. To overcome those losses, many of these
machines have been designed to use gravity for power.
Unfortunately, that just doesn't work. Like the saying
goes, "What goes up must come down."
There are other machines that improve
their efficiency by using other means of power. An
inventor recently came up with an automobile with a
large flywheel. When the car went downhill, the flywheel
would gather energy to be used when the car went uphill.
But it still needed an engine to make up the difference
of what was lost due to the resistive forces.
In conclusion
Simple machines usually exchange using a
smaller force over a greater distance to move a heavy
object over a short distance. The work required is the
same, but the force required is less. The are also
simple machines that help to reduce the resistance of
friction or such. |
