INTRODUCTION - SIMPLE MECHANICS AND WAVES
There is a theory in elementary physics that you might have seen. It says that it is possible to study the dynamics of a "dumbbell" rotor, of the type shown in Figure 1, by a simple transformation. The two masses rotating about their centre of gravity can be replaced by a single mass, called the "reduced mass", rotating on a circle with a radius equal to the distance between the original masses as shown in Figure 2. The reduced mass is given by the equation:
Now why would we wish to do such a thing? The answer as follows: We can think of Figure 1 as representing a diatomic molecule; the values of m1 and m2 are the weights of the two atoms and r is the distance between them: a total of 3 parameters. After the transformation we have one less parameter, so any calculations on the physics of the system become somewhat simpler.
You will go through the steps involved in obtaining an experimental value for the bond length in a diatomic molecule from its microwave (rotational) spectrum.
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