What are simple machines?
Name some machines people use. You might think of a computer,
a lawn mower, or a car. Those machines have lots of parts.
To a scientist, a machine is anything that helps us do work. A pulley is a
simple machine that helps you move objects. A simple machine has only a
few parts. Other simple machines are the lever, the wheel and axle, the
inclined plane, the screw, and the wedge.
Levers
Have you ever used a screwdriver to open a can of paint? If so, you have used a lever.
A lever has two parts. It has a bar or plank and a fixed point called a fulcrum. The
fulcrum supports the bar and allows it to turn, or pivot. The diagram below shows the
three kinds of levers.
Load and Effort Force
To describe how levers work, scientists use two other terms. The object
being moved by a lever is called the load. The force used to do work is
called the effort force.
A lever doesn't make you stronger. It changes how much effort force
you use to move a load. What happens if you apply a small force over
a long distance? The lever applies a strong force over a short
distance. This happens when you use a lever to pry open a can or jar.
You can also use a lever to apply a force over a small distance. The
lever then applies a weaker force over a distance. This happens when
you hit a tennis ball with a racket.
What are two other simple machines?
Wheels and axles are everywhere. Pulleys are less common. Wheels and Axles
When you turn a doorknob, you are using a wheel and axle. A wheel and axle
has two parts. You probably guessed that one part is a wheel. The other part, the
axle, is a bar that passes through the wheel's center. The bar is fixed to the
wheel.
Wheels and axles help you move things. You only need to use a small effort force on
the wheel. The axle turns it into a larger force. The larger force moves the load.
Have you ever been on a Ferris wheel? The cars are attached to
a giant wheel. The wheel hangs axle. When a force is applied to
the axle, it turns the wheel. The axle moves only a small distance,
but the wheel moves a lot more. The work of this machine gives
us pleasure! Fixed Pulleys
As you have seen, a pulley makes it easier to lift a load. There are different kinds of
pulleys.
In a fixed pulley, the wheel is attached to something. It cannot change position.
One end of the rope is passed around the wheel. The other end is tied to a load.
Pulling the rope lifts the load.
Other Types of Pulleys
A fixed pulley simply changes the direction of a force. You pull down.
The load moves up. This kind of pulley cannot change the amount of
effort force needed to move a load. The rope pulls on the object with
the same force you pull on the rope.
A movable pulley is not attached to a fixed object. It moves in the same
direction as the load. A movable pulley reduces the effort force needed to
move a load.
What happens if you hook a fixed pulley to a movable pulley? Look at the
diagram. The third drawing shows the answer.
What are inclined planes?
Suppose you are moving a heavy desk onto a truck. Would you lift the desk to get it on
the truck? That would take a lot of effort, and probably a helper or two. A better plan
would be to push the desk up a ramp.
A ramp is an inclined plane. An inclined plane is a simple machine with one part—a flat,
slanted surface. This machine holds up, or supports, most of the load's weight.
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A screw is an inclined plane wrapped around a cylinder.
Inclined planes make it easier for people with physical disabilities to get
around. You find inclined planes at the corners of sidewalks. They are often
at the entrances to buildings.
With an inclined plane, you only need a small effort force. However, you have to move
the load farther. Look at the photo. The cable must pull the car up the entire length of
the ramp. The ramp is longer than the distance needed to lift the car straight up.
An inclined plane is a flat, slanted surface.
Wedges and Screws
What do you get if you put two inclined planes back to back? You get a simple
machine called a wedge (WEJ). A wedge changes a downward or forward force
into a sideways force.
A screw is an inclined plane twisted into a spiral. To see how, cut a piece of
paper into a right triangle. Wrap the triangle around a pencil. Watch how the
slanted edge of the triangle goes up and around the pencil.
Like all inclined planes, wedges and screws trade distance for effort force. Often,
we use them on a very small scale. We use knives to cut food. We use drills to make
holes in wood and metal.
How do simple machines work together?
Two or more simple machines form a compound machine. Most machines are
compound machines. Scissors, for example, are made of two wedges. The
wedges act as firstclass levers. When you push the handles together, the blades
cut.
Bicycles combine all kinds of simple machines. The handle bars, pedals,
and brake controls are levers. The gears and chain are part of the axle that
moves the wheel. Screws of different sizes hold the parts in place.
Efficiency
A machine can save you time and energy. Yet no machine can change all the
energy of an effort force into work. Only some of the energy that comes out of a
machine is useful.
Efficiency (i•FISH•uhn.see) means how much work a machine produces
compared to the amount of work applied. Friction reduces the efficiency of any
machine.