Physics 105, Fall Term, 2009
Did you complete the reading assignment?
1) Work is a scalar
2) Work only depends the parallel
component of the force to the change in displacement of an object.
3) The work done on an object is equivalent
to the change in that objects kinetic energy.
Work can be a confusing topic for many people. The reason why most people
have trouble understanding work is because of the "parallel component"
clause. What this means is that any force that is not being applied to the
direction the object is moving is not contributing to the motion of that
object; therefore, it is not doing any work on that object. An example of
this is picking up a box and moving it to another location. It takes a
constant force to hold the box off the ground, but holding it off the ground
is not actually contributing to moving the box horizontally to a new
location. In this situation it takes work to lift the box, but it does not
take work to move the box once it has been lifted because the force of
holding the box and the motion of moving the box are perpendicular to each
Go to the following applet:
recognize your favorite man, file cabinet, and brick wall barriers waiting
for you there but in a new situation.
Turn off the friction by clicking the 'Frictionless' check box. This will
make it easier to observe what is happening with work for an introduction.
Open the 'Parallel Force Graph' and the 'Work Graph' if they are not already
visible. You will need to press the 'Clear' button on the left hand side to
examine new graphs for each activity.
Apply a large force on the file cabinet into the wall. The graph shows that
a force is being applied but that there is no work, why?
The cabinet is not moving.
The motion of the cabinet is perpendicular to
the force acting on it.
There is no force acting on the cabinet while
it is moving.
There is no opposing force to the movement of
Apply a large force to the cabinet for a short moment by clicking on it and
quickly dragging it with the mouse to the right or left and then releasing
the mouse so the cabinet goes up the ramp without being pushed while on the
ramp. What forms of work exist (are not zero) during this time? (check all that apply)
Explain the absence of any forms of work during this time.
Frictional work is not there because we made it a frictionless surface. In real situations there would be frictional work present.
What happens to the net work as the file cabinet goes up the ramp?
It stays constant
Why does the net work decrease as it goes up the ramp?
Gravitational work increases as it gets further from the ground, but no more force is being applied than what was originally there.
Now apply a large constant force on the block as it goes up the ramp. The
easiest way to do this is by entering a number in the 'Applied Force' box on
the left hand side of the applet and pressing 'Go'. Complete a trial at 100
N of applied force and one at 500 N of applied force. Pay special attention
to the net work. What is different? Why?
The net work decreases as the file cabinet goes up the ramp when 100 N of force is applied, but goes up when 500 N of force is applied. This is because the 100 N of force applied along the parallel distance is less than the gravitational work taken out of the system, while the 500 N of applied force over the parallel distance moved is greater than the gravitational work taken out of the system.
Now increase the applied work to 1000 N and complete a trial at a 10°
incline and at a 30° incline. Pay attention to the applied work and the net
work as the cabinet is pushed up the incline. Which form of work increases
as the angle of inclination increases?
Explain why one increases and one decreases.
Applied work increases because it is opposing a larger gravitational work as the incline increases. Net work decreases because the increase in gravitational work due to the incline takes more work out of the system.
Now open up the 'Energy Graph'. Make sure the scale for the y-axis is the same
for both the energy graph and work graph. Apply a constant force of 1000 N to
the file cabinet up a 10° incline. Which form of energy is the exact same as the
net work on the file cabinet?
The comments entered in these last two boxes go into a big anonymous data file that I will use to guide the lectures of the day. That means two things:
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answer you will need to come by the office or send an email.
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Was there anything that you didn't understand in the reading assignment?
What was confusing to you?