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11.3 Simple machines
Why do you think the
Ancient Egyptians used
ramps to help them
carry stones to the top
textbooks nis edu kz
of the pyramids?
110
Mechanical advantage
Simple machines are machines that do not need an internal source
of energy. To make them work, an external force needs to be exerted.
Some of these machines are able to multiply the input force, at the
expense of distance, to perform the work to be done. Simple machines
work with some combination of four basic principles:
1. increasing the magnitude of a force
2. increasing the distance or speed of a force on which it is acting
3. transferring a force from one place to another
4. changing the direction of a force
But remember, energy cannot be created or destroyed. This means
that the overall work done does not change. So a small force is able to
perform amazing feats of strength! The downside is that this force will
need to be applied for a much longer distance. This is the exchange
involved in simple machines everywhere: you swap a large force over a
small distance for a small force over a large distance.
This trade-off is what is known as the ‘golden rule of mechanics’: what is
lost in force is gained in displacement, and what is gained in force is lost
in displacement . We tend to use ramps, pulleys and other machines to
lift things that would otherwise be too heavy for our own muscles. As
we will see below, a smaller force achieves the necessary work, but over
a greater distance. These simple machines are designed to change the
transfer of energy from the work done due to the force exerted into
work done that requires a smaller force to perform the same task.
F 1
l 1 l 2
F 2
A gain in force for any lever can be calculated as a ratio of force applied
to force obtained.
Gain in force = F 1 = l 1
F 2 l 2
There are 6 basic types of simple machines in engineering: lever,
inclined plane, pulley, wheel and axle, and wedge and screw.