#$&*
course phy 231
Brief rotating strap and magnetsUses metal strap, magnets, threaded rod with bolts and washers, die
See also Pictures related to Straps and Toy Cars
Setup A: Using the TIMER, time the strap as it rotates to rest about the threaded rod, clicking with each 180 degree rotation. Click also when the strap comes to rest, and estimate how many degrees it has rotated since its last full 180 degree rotation.
Setup B: Repeat, but this time, with a ceramic magnet on each side of the axis of rotation, halfway between the axis and the end of the strap.
Setup C: Repeat once more but with the magnets positioned at the ends of the strap.
Setup D: Same as Setup A, except that the strap is taken off the threaded rod and placed on the die.
Setup E: Same as Setup B, except that the strap is taken off the threaded rod and placed on the die.
Setup F: Same as Setup C, except that the strap is taken off the threaded rod and placed on the die.
Report your data for Setup A (simply copy and paste from the TIMER output).
****
Start 2.4
First 180: 2.98
Next: 3.6
4.21
5.09
6.1
7.6
9.28-went to about 160 degrees, then stopped.
#$&*
Report your data for Setup B.
****
Start: 1.25
First 180: 2.01
Next: 2.81
4.13
5.72 to stop-went about 45 degrees
#$&*
Report your data for Setup C.
****
Start: 13.21
First 180: 13.91
Next: 14.74
15.72
16.63
17.8
19.29
22.91-pretty much stopped at 180
#$&*
Report your data for Setup D.
****
Start: 9.914
First 180: 10.306
Next: 10.754
11.306
12.183
13.842-stopped around 45 degrees after last 180 degrees
#$&*
Report your data for Setup E.
****
Start: 3.31
First 180: 3.56
Next: 4.01
4.629
5.04
5.67
7.58-went about 150 degrees after last 180
#$&*
Report your data for Setup F.
****
Start:7.125
First 180: 7.565
Next: 8.029
8.541
9.037
9.533
10.165
10.869
11.792
12.589
13.741
15.43
19.47-stopped about 165 degrees after full 180 degrees
#$&*
"
Analysis:
Now suppose your data for set A was
0.592
0.775
0.923
1.762
This implies four time interval, the first running from clock time 0 to clock time 0.592 sec, the second from 0.592 sec to
0.592 sec + .775 sec = 1.57 sec, the third from 1.57 sec to 1.57 sec + 0.923 sec = 2.50 sec, and the fourth from 2.50 sec to
2.50 sec + 1.762 sec = 4.26 sec.
The midpoint clock time of your second interval is halfway between 0.592 sec and 1.57 sec, at about 1.08 sec.
You could easily calculate the midpoints of all four time intervals, and should do so.
Report your midpoints below, based on your own data rather than the sample data used in the explanation:
****
#$&*
Each of your the time intervals in the sample data
0.592
0.775
0.923
1.762
corresponds to a 180 deg rotation. So for each interval you can easily find the average rate of change of angular position
with respect to clock time. Find the average rates and report them below. Be sure to include a detailed calculation, with
explanation, for one of your intervals.
****
#$&*
The moment of inertia of the strap itself is 1/12 M L^2, where M is its mass and L is its length. Assume the strap mass to be 70 grams.
The moment of inertia of a magnet attached to the strap is approximately M R^2, where M is the 50 gram mass of the magnet and R its distance from the axis of rotation (the axis of rotation is the threaded rod).
The total moment of inertia of the system is the sum of the moments of inertia of its components.
Find the moment of inertia for system A and report below:
****
#$&*
Report below a table of average rate of change of angular position vs. midpoint clock time, for the four intervals you
observed for Setup A. Report also the moment of inertia:
Setup A
****
#$&*
You are going to make the same report for the remaining systems. You can save yourself some time by using your calculator or Excel to do the necessary calculations.
Find the moment of inertia for the system in Trial B, and show your calculation in detail:
****
#$&*
For each system report average rate of change of angular position vs. midpoint clock time, and also report the moment of inertia:
Setup B
****
#$&*
Setup C
****
#$&*
Setup D
****
#$&*
Setup E
****
#$&*
Setup F
****
#$&*
Note that average rate of change of angular position is also called angular velocity. So you have calculated a number of angular velocities.
Sketch a graph of angular velocity vs. midpoint clock time for each setup. Describe how well each graph can be 'fit' by a single straight line. In a good 'fit' the points will appear to be randomly scattered about the line.
****
#$&*
On the whole, does it seem plausible that for these systems, the angular velocity tends to decrease linearly with time?
****
#$&*
Good. See instructions for analysis above.
#$&*