SIMPLE MACHINES
The machine makes it
easier by transferring the quantity of force applied, and the direction of the
force or both.
Simple
machines
are tools which do work with one motion.
There are two kinds of work that run a machine:
In an ideal machine, the incoming work is equal to the outcoming work. For the ideal machine:
Information
:
Force
working on the machine is called effort force (Fk).
The force produced by the machine to overcome resistance is called resistance force (Fb).
Part of the simple machine which is working the effort force is called
effort arm (dk),
Part of the simple machine which is working the resistance force is
called resistance arm (db).
1.
LEVER
The lever is a simple machine made with a bar
free to move about a fixed point called a fulcrum. The lever consists
of three parts :
a.
A fulcrum is the fixed point
where the rod turns freely,
b.
A load is the thing that the
lever lifts or moves and,
c.
An effort is the pushing force to
make the objects (load) move.
The position of the fulcrum
will determine the amount of the effort that is needed to lift an object.
Fulcrum is placed near the load,
less effort is needed to move the object.
Fulcrum is placed far away
from the load, more effort is needed to move the object.
Levers
are classified into three classes. The First Class Lever, Second
Class Lever and the Third Class Lever. The only difference between
them is between the fulcrum, effort and load, which one is located in the
middle of the system.
The
First Class Lever
In the First Class lever the fulcrum is located in the middle. The load is on one end and
the force is on the other.
Second Class Levers
Second Class Lever the fulcrum is on the end,
the resistance force, the load, is in the middle, and the effort is at the other end.
The Third Class Levers
The Third Class Lever, fulcrum is on the end of
one side, the effort force is in the middle, and the resistance force, the load is
at the other end.
The Mechanical Advantage (MA)
The number indicating the force multiplication done
by the machine is called a mechanical advantage.
2.
PULLEY
A pulley
is a simple machine consisting of a wheel with a grooved rim in which a pulled rope
or chain is run to change the direction and application of a pulling force.
Combination of Pulleys
Mechanical advantage in a system of pulley can
be measured by counting the sum of rope that
pulls a load upwards, to direction of pointer.
3.
INCLINED
PLANE
Using
an incline plane, the force needed to move the box to the back of the truck is
reduced compared to lifting the box straight up.
A knife, a screw, and a wedge are examples of special inclined planes.
SCREW
A screw is an inclined plane which is rotated to a cylinder.
WEDGE
A wedge is an inclined plane with one or two
inclined sides.
BIBLIOGRAPHY
Direktorat.2008. SCIENCE for GRADE VIII.
Jakarta:direktorat.
