Whenever you move something, you are doing
work. Lifting a pencil is a lot less work than lifting a desk. This is because
you use less force, or strength, to lift the pencil.
is not the only thing that is important in doing work. The distance, or
how far, something is moved is also important.
work needs a lot of force. And some work is done over a long distance. Because
of these things, work can be hard for people to do. So, people may use machines.
Machines are things which help make work easier.
people think of machines, they often think of cars or buses. But even a fork
is a machine.
Machines are made up of just a few kinds of simple machines.
kind of simple machine is the inclined plane. Inclined planes let you
use less force when you move something. One kind of inclined plane is a
staircase. A hill is also a kind of inclined plane.
point on a tack is a simple machine. It is called a wedge. Wedges are
used to push things apart. They may also be used to cut things. A knife is
another kind of wedge. Other kinds of wedges are a fork, plow, and needle.
jar with a top you have to twist on and off is using another simple machine.
The part of the top that holds it to the jar is a simple machine called a screw.
Besides jar tops, there are other kinds of screws used to hold things
together. Look at a door. Screws hold the door to the wall. The handle or knob
is held on by screws. Screws can also be used to lift things. The seats of
some chairs have screws. When the seat is turned, it may go up. Another way in
which screws are used is to push or pull things. Propellers on boats are
screws which push against the water. Propellers on airplanes are also screws.
trying to lift a desk without even touching it. Do you think you can do it?
You can, if you use a lever. A lever is another kind of simple machine.
When using a lever, it is often helpful to think of it as having three parts.
Somewhere between both ends of the lever is the fulcrum. The fulcrum is
the point on which the lever rests. On both sides of the fulcrum, there is an
“arm.” One arm has the load, or what you want to move. It is called the load
arm. The other arm is the arm you force down. It is called the force
arm. A seesaw is a type of lever, where the fulcrum is in between the
force and the load. Some other levers have the load in between the force and
the fulcrum. A wheelbarrow works like this. Another kind of lever has the
force in between the fulcrum and the load. Tweezers work like this.
simple machine is the wheel and axle. This machine is made up of a
wheel and a rod. The rod is the axle. It is joined to the center of the wheel.
The steering wheel on a car and handlebars on a bicycle are two kinds of wheel
and axle machines. A doorknob is also a kind of wheel and axle. Sometimes the
wheel on a wheel and axle has teeth. This kind of wheel is called a gear.
A gear can also turn another gear. Gears are used this way inside of watches.
you use a lever, you use force in one direction. The load moves in another
direction. Another simple machine works in this way. It is a pulley. A
pulley is made up of a wheel and a rope. When you use a pulley, you use force
in one direction. The load then moves in another direction. Many times it is
hard to get at something to move it. For this reason, you might want to use a
pulley. Pulleys can help raise flags on high poles. They can help lift things
out of water. Most often, a pulley by itself cannot help you use less force to
move something. But two pulleys can. Three, four, or even more pulleys can be
used together to help make work still easier. Pulleys joined together in a
certain way are known as a block and tackle.
All machines are made up of simple machines. Most machines you see and
use every day are made up of two or more simple machines. These machines are
called compound machines.
compound machine that you probably use is a pair of scissors. Scissors may
seem simple. But one pair is made up of two levers. They are joined at the
fulcrum. On each lever there is also a wedge used for cutting. Another
compound machine is the pencil sharpener. It has wedges, screws, wheel and
axles, and gears all working together.
Using Compound Machines
Building and road workers have to dig down into the ground and move
rocks and earth. They may use a bulldozer or a power shovel to do these
compound machines are also used around homes and in parks. Each summer, grass
is cut, trees are cut down, and bushes are moved and planted. In the fall,
leaves may have to be taken away. In winter, snow and ice may have to be taken
away from streets and sidewalks.
compound machines can help people do work, they can also help people have fun.
Machines are often used in sports. When people fish, they may use a fishing
rod and reel. A fishing rod and reel is a compound machine. Bicycling and
skiing are other sports in which compound machines are used.
toys and games are also made of compound machines.
When you lift something, you use your force to lift against another
force. That other force is called gravity. Gravity is the force which
pulls things to the earth.
you drop your pencil, the force of gravity pulls it down. When you pick it up,
you use your force to lift against gravity.
use many machines to work against gravity. Inclined planes, elevators, and
even space rockets are such machines.
Pushing and Pulling
One reason why you push and pull things is to start them moving. Things
cannot move by themselves. All things will stay still until a force causes
them to move.
as a force is needed to start something moving, a force is needed to stop
something from moving. So you may have to push or pull something to stop it
you were to push or pull a box across the floor. The box would rub against the
floor. This rubbing is called friction. It is friction which would make
the box slow down and stop. To keep the box moving, you would have to keep
pushing or pulling it.
have learned a way to get less friction when they push or pull things. They
put loads on things that have wheels. Carts and wagons are such things.
can wear things away. Many machines have moving parts. As these parts move
they rub against each other. Friction can wear away these parts of machines.
People who use machines have learned another way to make less friction. They
put oil on the moving parts of machines.
can be good. Riding a bicycle downhill and coming to a stop, you used your
hand or foot to press the brakes. The brakes rubbed against the wheel. This
rubbing caused friction which helped you stop. Pouring sand on icy roads
causes friction when walking or driving a car.
Steam turbines are not used for driving a car or airplane. They are too
heavy, and they take too long to “get up steam” and get ready to go. But
there is another kind of engine that does not weight so much and that starts
quickly. For cars and planes a gasoline engine works well. The energy of this
engine comes from burning gasoline.
gasoline or any other fuel burns, it gives off hot gases and it heats up the
air near it. If the fuel burns in some kind of container, the molecules of
heated-up air and gases press on the sides of the container.
the fire of a little match can make a powerful pressure.
are several cylinders in an engine, made of metal. The “stopper” is a
round metal part called a piston. Each piston has a rod that is joined to a
crank. Each piston is pushed by the pressure of hot gases from a gasoline
after the other, gasoline fires keep popping and pushing against the pistons.
A special part keeps mixing more gasoline and air to burn.
pressure of the hot gases gives the force to turn the wheels of a car or the
propeller of an airplane.
Some trucks, trains, and boats are run by diesel engines. A diesel
engine is like a gasoline engine in most ways. Its heat energy comes from
burning fuel, but it burns oil instead of gasoline. Diesel engines are good
for pulling heavy loads.
Gasoline and diesel engines burn their fuels inside themselves. Another
word for inside is internal. Another word for burning is combustion.
Engines that burn fuel inside themselves are called internal combustion
Gas Turbine Engines
Scientists have worked out a kind of heat engine that uses hot gases of
a flame. In a gas turbine a liquid fuel is burned. The hot gases from the
burning fuel have a great deal of force. They push against a turbine and make
it turn. The gas turbine can do work by making a truck, locomotive, or ship
turbine engines are used in some airplanes. The engines turn propellers.
An electric motor has a wheel which is turned by an electric current.
When electric current flows through a wire, the wire becomes a magnet. An
electric current has energy.
every electric motor, there is an electromagnet. There is also an electric
current to operate the electromagnet and something for the electromagnet to
motors have a part that starts and stops the electric current like a very
quick touch and take-away.
electric motors do not need such a part. Such electric motors work on a kind
of electric current called A.C. this stands for alternating current.
Alternating current acts like the touch and take-away. Alternating current
makes the electromagnet of a motor go on and off.
Electricity for homes and factories comes from machines called generators.
When a generator is turned, electric current flows through wires to places
near and far.
electricity may come from a big generator many miles away. On a farm it may
come from a small generator in the tool shed.
some places the force of flowing water turns a turbine. The turbine turns a
generator. In other places a steam turbine does the work of turning the
generator. Small farm or home generators are turned by gasoline or diesel
important thing is that the generator does not have to be in the same place as
the things which use the electrical energy.
do not have to build our factories, stores, or homes next to a river or a
steam turbine. We can build them any place that suits us. We can string wires
from the electric generator to the buildings where we use the electrical
electric current turns the motors in our vacuum cleaners, electric
refrigerators, and many other machines. It gives light and heat where we need
them. Forces from far away supply energy where we need it.
Machines can help people do many kinds of work. There is even a machine
which can run other machines. This machine is called a computer.
can run the machines that make things. They can run machines that make
clothes. Many books are made with the help of a computer.
computers can run other machines, many people are needed to help run
computers. Computer programmers are people who set up computers to do a
certain kind of work. These people may “teach” a computer how to run other
operators are people who run computers. They make sure a computer is
working the way it should.