NONSTANDARD EXPERIMENTS IN INTRODUCTORY PHYSICS LABORATORY

ENERGY CONSERVATION IN ROTATIONAL MOTION OF RIGID BODY ON A TURN TABLE

If you are interested, visit Introduction to Newtonian Mechanics with Java Applet Simulated Experiments.

Abstract

This is an extension of popular Introductory Physics Lab experiment determining inertia moment of a rotating body. Here, however, kinetic, potential and thermal energies related to the rotating body are analyzed. Due to considerable friction torque, mechanical energy is not conserved but mechanical and thermal energies counted together are. Data analysis is performed with help of automated spreadsheet presenting results in tabular and graphical forms.

Introduction

Introductory Physics Lab experiments or demonstrations showing mechanical energy conservation with help of air tracks or properly made pendulae and photogate systems are probably quite common. This experiment is an extension of well known experiment where inertia moments of rotating bodies are determined. Due to friction torque considerable part of mechanical energy in such experiments is dissipated in a form of thermal energy. If it is known how velocity of any point of the rotating body changes in time, in principle, a magnitude of frictional torque can be found and this thermal energy can be calculated. A working arrangement for such measurements is shown in the THEORY sheet in Fig. 1. In this sheet a theoretical background for such calculations is explained in details.
The calculations themselves are not difficult but rather lengthy. This is where the automated spreadsheet CALCULATIONS becomes very helpful. For user comfort important calculation steps are explained and referred to the theory. Intermediate and final results are displayed in a form of automated tables and graphs. The user have to deliver there only required experimental data typing or pasting (Paste Special, Values only) them into the yellow fields. If the employed photogate software can deliver velocity vs. time data that can be displayed in generic word processor (e.g. Notepad or Write) it may be copied from there.
Exactly what kind of data are required is well visible in the image of CALCULATIONS sheet below THEORETICAL BACKGROUND. The real Excel spreadsheet can be downloaded as EnCons.zip. To have good data quality a too narrow fin must be not used. Most probably its width should be between 3 and 5 cm. Also an accuracy of used photogate system should be near 10-4 s. In our lab three different kinds of turn tables made by CENCO were tested by students. A few hours before use the turn tables were lubricated. All of them worked well, but one made probably sixty years ago was exceptionally fine.
CALCULATIONS sheet can be copied and pasted into exactly the same place in any other sheet. The graphs, however, may still take data from the original sheet. If they do, to force them to use data from new sheet click right on the graph far enough from any label. Select from popup menu Source Data and change in data references the name of the original spreadsheet to the name of actual spreadsheet in all series of data.

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