Photo Shows Demonstration Used by the University of Texas at Austin.
Concepts Conveyed:
Provides a visual of how a torsion pendulum works, and how changing the torsion constant and/or moment of inertia affects the frequency of oscillation.
Demo Staff:
The entire torsion pendulum apparatus should already be set up as is, except for the beam that is to be twisted.
First, select the beam that corresponds to material that the professor wishes to use for the
demo. There is a black base that will hold the beam securely in place. Note that there are 2
holes in the base: this will be for securing the beam’s platform with screws.
The beam has a metal platform at its base. Note that there are 2 holes in the metal platform. Align these 2 holes on the platform with that of the black base.
The metal platform that the bar is attached to.
Using the two metal screws provided, secure the beam’s metal platform to the base by passing
them through the 2 aligned holes and turning them clockwise. The beam should now be firmly
secured to the base.
The beam's platform secured to the base
The next step is to secure the top of the beam to the vertical bar above it. First, adjust the bar to an appropriate height as you see fit by untightening the black screw that attaches the bar to the pole. Make sure to align it such that the other end of the vertical bar is directly above the top of the beam.
There is a cylindrical metal tube at the end of the vertical bar. This will be used to hold the head of the beam in place. By loosening the black screw that is holding the cylindrical tube in place, allow it to slide down on top of the top of the beam. It should be resting on top of the Allen screw at the head of the beam.
The cylindrical tube that secures the head of the beam to the vertical bar.
Tighten the pin on the side of the cylindrical tube such that it is secured to the head of the beam.
Beneath the base, there is a circular protractor that measures the angular displacement of the torsion pendulum. Align the black line that goes through the diameter of the protractor such
that it is at 0 radians. Once this has been achieved, tighten the black screw that secures the
cylindrical tube to the vertical bar.
Upon delivery of the demo into the lecture hall, ensure that the 3B Scientific Electronic
counter/timer is plugged in and set to measure a frequency in Hertz. See Fig. 4 for reference.
Scientific Electronic counter/timer set to Hertz
Ensure that the photogate is on. The red light on the side of the photogate will be on if it is off. To turn it on, there is a black tab at the bottom of the photogate from which the laser will emerge. Slide the black tab to the left. The red light on the side of the photogate should now be off. See Fig. 5 and Fig. 6 for reference
Photogate turned off.Instructions/Notes:
Push the "Start/Up" button on the 3B Scientific Electronic counter/timer.
On the glass platform above the circular protractor, there are metal tabs. Use these metal tabs to displace the torsional pendulum from its equilibrium position. Displace it no more than 15 radians.
Release. The 3B Scientific Electronic counter/timer will begin measuring the frequency of
oscillation of the torsional pendulum.
When finished, press the "Stop/Down" button on the 3B Scientific Electronic counter/timer.
Then press the "Reset" button on the 3B Scientific Electronic counter/timer to reset the
reading.
Compare the frequency of oscillation with the beams that are made of different materials to
demonstrate the dependence of the frequency on the torsional constant of the material. See
instructions for Demo Staff or come in before using the demo to learn how to change the
beam.
There are 2 100g masses that come with the demo. One can place these masses on the metal
tabs on the disk to increase the moment of inertia of the torsional pendulum. Compare the
frequency of oscillation of the set-up with a higher moment of inertia to one with a lower
moment of inertia (without the masses) and note the dependence of the frequency on the
moment of inertia of the torsional pendulum.
Torsional pendulum with 2 100g masses added.
Table 1: The frequencies of oscillation for different materials at different lengths.
Steel
Brass
Copper
Aluminum
300 mm
N/A
3.6 Hz
4.1 Hz
5.4 Hz
500 mm
6 Hz
3 Hz
3.3 Hz
4.5 Hz
Table 2: The frequencies of oscillation for different materials with and without additional mass for a fixed length of 500 mm
Steel
Brass
Copper
Aluminium
0g
6
3
3.3
4.5
200 g
3.5
1.9
2
2.6
Table 3: The frequencies of oscillation for different material with and without additional mass for a fixed length of 300 mm.
Brass
Copper
Aluminium
0g
3.6
4.1
5.4
200 g
3.6
4.1
5.4
Last Updated 2024 - University of Texas at Austin Physics Dept.
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