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A
machine is a tool used to make work easier.
Simple machines
are simple tools used to make work easier.
Compound machines
have two or more simple machines working together to make
work easier.
In science, work is
defined as a force acting on an object to move it across a
distance. Pushing, pulling, and lifting are common forms of
work. Furniture movers do work when they move boxes.
Gardeners do work when they pull weeds. Children do work
when they go up and down on a see-saw. Machines make their
work easier. The furniture movers use a ramp to slide boxes
into a truck. The gardeners use a hand shovel to help break
through the weeds. The children use a see-saw to go up and
down. The ramp, the shovel, and the see-saw are simple
machines.
Inclined
Plane
A plane is a flat
surface. For example, a smooth board is a plane. Now, if the
plane is lying flat on the ground, it isn't likely to help
you do work. However, when that plane is inclined, or
slanted, it can help you move objects across distances. And,
that's work! A common inclined plane is a ramp. Lifting a
heavy box onto a loading dock is much easier if you slide
the box up a ramp--a simple machine.
Wedge
Instead of using the
smooth side of the inclined plane, you can also use the
pointed edges to do other kinds of work. For example, you
can use the edge to push things apart. Then, the inclined
plane is a wedge. So, a wedge is actually a kind of inclined
plane. An axe blade is a wedge. Think of the edge of the
blade. It's the edge of a smooth slanted surface. That's a
wedge!
Screw
Now, take an inclined
plane and wrap it around a cylinder. Its sharp edge becomes
another simple tool: the screw. Put a metal screw beside a
ramp and it's kind of hard to see the similarities, but the
screw is actually just another kind of inclined plane. How
does the screw help you do work? Every turn of a metal screw
helps you move a piece of metal through a wooden space. And,
that's how we build things!
Lever
Try pulling a really
stubborn weed out of the ground. You know, a deep,
persistent weed that seems to have taken over your flower
bed. Using just your bare hands, it might be difficult or
even painful. With a tool, like a hand shovel, however, you
should win the battle. Any tool that pries something loose
is a lever. A lever is an arm that "pivots" (or turns)
against a "fulcrum" (or point). Think of the claw end of a
hammer that you use to pry nails loose. It's a lever. It's a
curved arm that rests against a point on a surface. As you
rotate the curved arm, it pries the nail loose from the
surface. And that's hard work!
Wheel and
Axle
The rotation of the lever
against a point pries objects loose. That rotation motion
can also do other kinds of work. Another kind of lever, the
wheel and axle, moves objects across distances. The wheel,
the round end, turns the axle, the cylindrical post, causing
movement. On a wagon, for example, the bucket rests on top
of the axle. As the wheel rotates the axle, the wagon moves.
Now, place your pet dog in the bucket, and you can easily
move him around the yard. On a truck, for example, the cargo
hold rests on top of several axles. As the wheels rotate the
axles, the truck moves. You can move your dog across the
country!
Pulley
Instead of an axle, the
wheel could also rotate a rope or cord. This variation of
the wheel and axle is the pulley. In a pulley, a cord wraps
around a wheel. As the wheel rotates, the cord moves in
either direction. Now, attach a hook to the cord, and you
can use the wheel's rotation to raise and lower objects. On
a flagpole, for example, a rope is attached to a pulley. On
the rope, there are usually two hooks. The cord rotates
around the pulley and lowers the hooks where you can attach
the flag. Then, rotate the cord and the flag raises high on
the pole.
If two or more simple
machines work together as one, they form a compound machine.
Most of the machines we use today are compound machines,
created by combining several simple machines.
Can you think of creative
ways to combine simple machines to make work easier? Think
about it.
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