PHY 2048L
Lab 7

Moment of Inertia and Rotational Motion

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Updated: 1/21/2013
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We will be doing a computerized version of "Lab 17", so there will be significant differences between the pictures you see in the theory section (pages 179 to 181) and what you see at the bottom of this page.

Guiding Question:

We will test this by checking whether our measured moment of inertia for a pair of masses attached to our apparatus agrees with the theoretical prediction for two point masses.

Preparing for the lab:

Read the theory for "Lab 17" (pages 179 to 181) in the lab manual as well as the equivalent version on the lab handout before answering the questions on LON-CAPA.

Comments

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. A sample of the procedural part of that handout is linked here. (The post-lab questions used vary from year to year.)

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, but are mostly the same as what we did in Lab 3. This program automates the process you followed in the labs like the two 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.]

rotational motion apparatus connected to computer interface

You will wind a string around the medium-sized pulley on the top. A small mass will then provide the force (turned into a torque by the pulley) that gives the mass-and-rod combination an angular acceleration. There is a small hole you can use to tie the string to the pulley, or you can tie it around the smallest pulley and use a small notch in the medium pulley to keep it from slipping.

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.

 
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