What Is A Machine?
Think of the word machine and you may
conjure up (imagine) pictures of big bulky
cranes, bulldozers, or even a copying machine.
However, in science and engineering the word
"machine" has a very specific meaning.
A machine is any device that can apply
mechanical energy at one point and deliver
it in a more useful form at another point. A
machine is any device that provides a
"mechanical advantage".
Simple machines are
all around us and help us accomplish work faster and easier
many times each day.
Let's
use some examples to explain the concept of simple
machines and mechanical advantage.
You have a box with 120 pounds of books to
return to the library. A friend will drive you
to the library in their truck. You just have to
get the books into the bed of the truck. Wow!
That would be a lot of weight to lift using your
own muscles. You need a mechanical advantage!
Instead of carrying the box of books to the truck
you place them in a wagon or on a dolly and
pull the wagon (or dolly) to the truck. The
rotating wheels on these simple machines
make it easier to move the books. Next instead
of lifting the books from the ground to the truck,
you make a ramp out of a board and push the
box up the ramp into truck. The ramp is a
simple machine called an inclined plane.
Now you are ready for a soda! When you use a
bottle opener to pry the cap off of a soda bottle
or the pop top on a soda can, you have used a
simple machine. The bottle opener and pop tops
are forms of another type of simple machine called a lever.
How Do Machines Help Us?
There are many different types of machines
with varying capabilities and functions.
Different machines may:
- Transform energy (change from one type of energy to another type).
Example: a generator can change mechanical energy to electrical energy;
a steam turbine changes thermal (heat) energy to mechanical energy.
- Transfer (deliver) energy.
Example: an automobile's drive train transfers energy to the
rear wheels.
- Increase or multiply force. Example: pulleys can lift
more weight with less force.
- Increase or multiply the speed. Example: as you pedal a bicycle
you turn the bicycle's gears, however the
bicycle wheels move faster than the gears.
- Changes the direction of the force. Example:
you pull a flag pole pulley down to raise the flag.
- Reduce friction.
Common simple machines include the:
- lever
- inclined plane
- wheel and axle
- wedge
- pulley
- screw
- gear and belt
- cam and crank
- spring
- rotating wheel
Simple machines are typically used when:
- The amount (the magnitude) of the force
needed can not be applied without
the aid of a machine. This applies
to our example of the 120 pounds of books. The
books were too heavy to move without the
aid of two simple machines -
a wagon with rotating
wheels and an inclined plane.
- The direction of applied force is not
in the desired direction. For example, when you pull down
on the flag pole rope you want the flag to move in the
opposite direction; you want the flag to go up, not down.
A pulley is a simple machine that allows you to change direction
of a force.
Mechanical Advantage
Archimedes was a great mathematician
and engineer who was born in 287 BC in
Syracuse, Sicily. He is credited with the
development of many of our
modern day mathematical and mechanical
principles (such as Archimedes' principle,
the concept of pi
(p) and geometric proofs) and machines like the lever,
pump, and pulleys.
Archimedes understood the concept of
mechanical advantage and how to use it
to move or lift heavy objects with less force.
In mathematical terms the mechanical advantage (abbreviated as MA)
is the ratio of the load to the applied force.
MA = Load/Applied Force
A good mechanical advantage is greater than 1. The greater
the mechanical advantage the smaller the applied force needed to accomplish
the task.
Although machines use less force to accomplish the same task there
is a trade-off. Using the ramp required less
force than lifting those 120 pounds of books from the ground
to the bed of the truck. Although less force was
needed to move the box of books into the truck,
that reduced force must be applied for a longer period of
time or longer distance.
Inclined Planes
In the activity, you lifted a rock straight up.
It was heavy. When you pulled the rock up the
incline, you did not have to use as much force.
It was easier to move the rock up the incline than
to lift it straight up. However, the inclined plane
is longer than the height of the stack of books.
Less force was needed to move the rock up the incline
but you had to pull the rock a longer distance.
It is easy to spot the simple machine, the inclined plane, in
our daily lives. Some applications of inclined planes include:
- Ramps for wheelchairs
- Car ramps
- Playground slide
- Boat ramps
REFERENCES:
Freeman, Ira M. and Durden, William J., Physics Made Simple,
Doubleday, New York, 1990.
Halpern, Alvin, Schaum's Outline of Theory and Problems of
Beginning Physics I: Mechanics and Heat, McGraw-Hill, Inc., New York, 1995.
Macaulay, David, The Way Things Work, Houghton Mifflin Company, Boston, 1988.
