Experiment 13
ROTATIONAL DYNAMICS

PROCEDURE


CAUTION: IN ALL PARTS OF THIS EXPERIMENT, BE CAREFUL (ESPECIALLY AT END OF A RUN) THAT THE CORD DOES NOT RUN UNDER THE DRUM A.


1.  Open the Worksheet and fill in the header information.

2.  In this experiment, you will use the "smart" pulley to measure the linear acceleration of the system. Make sure the pulley is connected to the DataLogger interface and turn it on. Open the Pulley.rot link.


Correcting for Friction:

3.  It is essential to eliminate the effects of friction from the calculations in so far as possible. With nothing on the rotatable platform, determine the suspended mass (including the mass of the weight holder) that is just sufficient to cause the platform to turn at constant angular speed. Use the 5 g (small) mass holder in this step through step 9. For the empty platform, this mass should lie in the range of 5 to 10 grams. Record the amount of this mass in kilograms in cell D8 on the Worksheet. The weight of this mass is equal to the force required to overcome the frictional force. Leave this mass on the holder for the next procedure. If you change to a more massive holder, make sure that the total mass (including the mass of the old holder) that was used is always present on the new holder to overcome friction.


Moment of Inertia of the Platform:

4.  With nothing on the rotatable platform, use a driving mass of 30 g = 0.030 kg. Note that the driving mass is added to the total mass necessary to overcome friction. Hit the Collect button on the LoggerPro window. The data will automatically be collected once the motion starts.

5.  After the mass hits the floor, hit the Stop button on the LoggerPro window. The Velocity Graph will have all the data plotted. In order to determine the linear acceleration of the system, you need to find the slope (rate of change) of the velocity vs. time curve, which should be a straight line. The application will do this job for you, but we need to be selective about what points it should use.

6.  After the mass hits the floor, the data points should be thrown out since the system is not accelerating any more. Use the mouse to select only the linearly increasing portion of the data.

7.  Select the ANALYZE...Linear Fit menu to plot the best straight line through the data. The fitted line should pass through only the linear portion of the data. Record the value of the slope as it appears in the floating information box as the acceleration in cell C12. Also record the uncertainty associated with the fit in cell D12.

8.  Repeat the procedure for 2 more runs (3 in all), and record the acceleration values and the uncertainties in cells C13:D14. Determine the average acceleration and its uncertainty, and enter the result in cells C16:D16.

9.  Repeat the above procedure, except this time use a driving mass of 50 g = 0.050 kg. In the same way, collect acceleration data for 3 runs. record the acceleration values and the uncertainties in cells C20:D22. Determine the average acceleration and its uncertainty, and enter the result in cells C24:D24.


Moment of Inertia of the Platform plus Disc:

10. 

a.  Add the disk to the platform, determine the mass necessary to overcome friction as in step 3 above, and record the value in cell D28. This will most likely be somewhat different from the result in step 3, with the mass required lying somewhere between 10 g and 20 g. Be sure to record the ID number (if available) of your disk in cell F26.

b.  Change to the 50 g (large) mass holder. With the disk on the platform, repeat steps 4 through 8, except use a driving mass of 90 g = 0.090 kg. Take 3 trials and record the acceleration values and the uncertainties in cells C32:D34. Determine the average acceleration and its uncertainty, and enter the result in cells C36:D36.

c.  Repeat the above procedure, except use a driving mass of 160 g = 0.160 kg. Take 3 trials and record the acceleration values and the uncertainties in cells C40:D42. Determine the average acceleration and its uncertainty, and enter the result in cells C44:D44.

11.  Measure the diameter D, and mass M of the disk using the meter stick and scale in the lab, and record the values in cells C49 and C53, respectively. Calculate the radius R, and enter the value in cell C51 on the Worksheet.

12.  Use the vernier calipers to measure the diameter of the hub where the string is wound. Record the value in cell F49 on the Worksheet. Calculate the hub radius r, and enter the value in cell F51.