Updated: 8/23/2008
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Updated: 8/23/2008
This lab is a fully revised version of Lab 17 in the manual. You should read the theory section of Lab 17, but all of the procedures, data sheet, and post-lab questions are on a handout. The procedural part of that handout is linked here. (The post-lab questions used vary from year to year.)
Prelab
Answer questions 1 - 5 on pages 183 and 184. You should also review the handout and note what the post-lab questions ask you so you can think about them while doing the lab.
Comments
Most of the measurements in this lab will be made with the computerized data acquisition program Data Studio. Details on its use are given in a detailed handout. This program automates the process you followed in the labs that used the air table, converting measured (angular) positions into angular velocity or acceleration and displaying the results. It even puts them in the right units.
Note: The rotational motion apparatus includes a small box that simply puts out a pulse each time the axle rotates a fraction of a degree. The box attached to the computer feeds the number of counts in a fraction of a second to the "science workshop" program. The program interprets those counts to determine an angle, much as you count the number of millimeters on a ruler to determine a distance, but it does not interpolate. If it has not quite gotten to the next "mark", the angle will come up short. If there is a "mark" just after it starts counting, the angle will come out a bit large.
Be sure to measure the mass of the two weights that go on the rod before positioning them and measuring their radius. The "Setup" section (steps 2 and 3) makes this clear, but the fact that they will be preinstalled on the device leads some students to ignore the directions.
Pay attention to step 3 under "Collect and analyze the data". It is easiest if you first release the pulley and then click the Start button, but you can also start the timer first like we did in Lab 2 with the gliders on the air track. Similarly, it is easier if you click the Stop button before the mass hits the floor or you run out of string, but not necessary. You can always exclude those parts of the data when you do your fit. Finally, be sure the control panel window has "input focus" before starting to take data or it may not respond to the mouse when you click on Start.
Be careful to pay attention to the difference between diameter and radius.
There will eventually be some information on using the Data Studio tools on the graphical display when doing the fits. For now the only information is in the pdf file linked here and handed out in class showing the step-by-step procedure when doing this lab.
Apparatus
The rotational motion appartus (left) is connected to the
computer interface (right). The order of the connections
(that is, whether the yellow connector is on the left or right)
determines whether a particular rotation direction is recorded
as positive or negative.
[If your velocity and acceleration are negative, you can either
reverse the way you wind the string or you can reverse the plugs
to get them both positive.]
There are a few things you need to watch: The instructions tell you to put the second pulley at an angle so the string goes straight over the pulley. This is very important. You also need to be sure that this pulley does not rub on the mounting bracket. Any extra friction can be a major problem. Finally, avoid using too much string and watch how you wind it on the pulley. Slippage and/or a major change in radius can make the torque change with time and result in a non-constant acceleration.
Contact me
if you have any questions.