## Due 3:30 pm, Thursday, September 24th

Physics 105, Fall Term, 2009

Did you complete the reading assignment?

Yes
No

## KEY CONCEPTS:

1) Work is a scalar quantity.

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 other.

Go to the following applet: http://phet.colorado.edu/sims/the-ramp/the-ramp_en.jnlp. You'll 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 the cabinet.

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)

Applied work
Frictional work
Gravitational work
Net work

Explain the absence of any forms of work during this time.

What happens to the net work as the file cabinet goes up the ramp?

It increases
It decreases
It stays constant

Why does the net work decrease as it goes up the ramp?

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?

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?

Applied work
Net work

Explain why one increases and one decreases.

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?

Total energy
Kinetic energy
Potential energy
Thermal energy

FEEDBACK

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:

1) If you have a specific question or concern and would like an individual answer you will need to come by the office or send an email.
2) If you really want to get an anonymous comment to me before the lecture, it must be submitted before 11am that day.

Was there anything that you didn't understand in the reading assignment?  What was confusing to you?