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Workwork is done in physics when a force is applied on an object and there is a
displacement parallel to the direction of the force. No work is done unless there is a displacement parallel to the direction of the force. The force doing the work could be a component of the applied force. It would be the component parallel to the displacement.
The general form of the work equation is:
W = F d
where W is the work, F is the force acting in the direction of the displacement, d
Here is an example of a situation where a force is applied and no work is done: A man holds a 50 N weight. No work is done (even though he must exert a force to hold the weight) because there is no displacement parallel to the direction of the weight.
Here is an example of a situation where there is a force applied and there is displacement and no work is done against the object's weight: A man walks around the room holding a 50 N weight. The object's weight acts down. For work to be done against the weight there has to be displacement in the direction of the weight (either up or down). There is none so no work is done against the weight. There is work done against friction as the man walks around the room.
Positive work -the displacement and the
force are in the same direction.
Negative work - displacement and the
force are in opposite directions.
In the drawing, F is applied at angle q to
the horizontal. The force doing the work is the horizontal component of
the force since it is the component that is parallel to the horizontal
displacement. In this type of situation, the form of the work equation
W = F cos q d
Joulethe SI unit of work.
1 J = 1 N m, or one Joule equals one Newton meter
Work can be represented graphically. If you plot force vs displacement, the
area under the curve represents the work done.
Calculus application: Integration is used to find the area under a curve. If
you integrate this curve, you have found the work done.
Powerrate of doing work.
P = W / t
where P is the power dissipated, W is the work done, and
t is time in seconds
Wattsthe SI unit of power
1 W = 1 J/ 1 sec, or one watt equals one joule per second
MachineA device that changes the force doing the work. A machine multiplies your force, not work. A machine allows you to apply less force, but you apply it over a greater distance. No machine is 100% efficient; there is always work done against friction.
Simple machineThere are six types of simple machines. A compound machine is simply a combination of one or more simple machines.
Types of simple machines:
Work inputthe work you do
with the machine.
Wi = (force that you apply)(distance you apply it over)
- inclined plane
- wheel and axle
Work outputthe work the machine does for you
Wo = (weight of the object)(height object is raised)
Efficiencythe ratio of work output to work
input. Efficiency gives you an idea of how much work is being lost in
overcoming friction. In an ideal machine with no friction, the efficiency
would be 100%. (I abbreviate efficiency as "eff") In our world containing friction, work output always is less than work input.
eff = Wo / Wi
Ideal mechanical advantagetells you
how much the machine would multiply your effort force (the force that you
apply) by if there were no friction; abbreviated IMA
IMA = d / h
where d is the distance that you apply your force over and
h is the height the object is raised
Mechanical advantagetells you how much
the machine multiplies your effort force by with friction; abbreviated MA
MA = W / F
where W is the weight of the object and F is the force
that you apply
Efficiency can also be expressed in terms of advantage:
eff = MA / IMA
Work Sample Problems
Energy Sample Problems