http://www.ck12.org Chapter 8. Work, Power, and Simple Machines
(Effort Force)(Effort Distance) = (Resistance Force)(Resistance Distance)
Theeffortis the work that you do. It is the amount of force you use times the distance over which you use it. The
resistanceis the work done on the object you are trying to move. Often, the resistance force is the force of gravity,
and the resistance distance is how far you move the object.
Theideal mechanical advantageof a simple machine is the ratio between the distances:
IMA=resistance distanceeffort distance
Again, the IMA assumes that there is no friction. In reality, the mechanical advantage is limited by friction; you must
overcome the frictional forces in addition to the resistance force. Therefore, theactual mechanical advantageis
the ratio of the forces:
AMA=resistance forceeffort force
When simple machines are combined to form compound machines, the product of each simple machine’s IMA gives
the compound machine’s IMA.
Simple Machines
Lever
Aleverconsists of an inflexible length of material placed over a pivot point called afulcrum. The resistance is the
object to be moved (shown here in red), and is placed to one side of the fulcrum. The resistance distance in a lever
is called the resistance arm. The effort is exerted elsewhere on the lever, and the effort distance is called the effort
arm or effort lever arm. The lever shown here is the most common type of lever, a Class One Lever, but there are
two other types of levers. If you would like to learn about the other types of levers, visit this website:
http://www.ohio.edu/people/williar4/html/haped/nasa/simpmach/lever.htm
The effort work is the effort force times the effort lever arm. Similarly, the resistance work is the resistance force
times the resistance lever arm. If we ignore any friction that occurs where the lever pivots over the fulcrum, this is an
ideal machine. Suppose the resistance force is 500. N, the resistance arm is 0.400 m, and the effort arm is 0.800m.
We can calculate exactly how much effort force is required to lift the resistance in this system:
Output Work=Input Work
(Resistance Force)(Resistance Arm) = (Effort Force)(Effort Arm)
( 500 .N)( 0 .400 m) = (x)( 0 .800 m)
x= 250 .N
In this case, since the effort arm is twice as long as the resistance arm, the effort force required is only half the
resistance force. This machine allows us to lift objects using only half the force required to lift the object directly
against the pull of gravity. The distance the effort force is moved is twice as far as the resistance will move. Thus,
the input work and the output work are equal.