I came across a slightly more advanced variation of this problem in a mechanics course earlier last semester, and for some reason I just couldn’t figure out how to find the kinetic energy. I found it instructive to go through a quick calculation…
The system is shown above – a simple rod swinging around a pivot that goes through one end. Assuming the usual things (the pivot at exactly the end of the rod, for example), there are two ways of finding the kinetic energy.
1. Shifting the Moment of Inertia
This is the classic way to approach this problem. Rotational kinetic energy is defined in terms of the moment of inertia about the pivot:
so we just use the parallel axis theorem to get the moment of inertia of a rod about its end,
where is just the distance between the center of mass and the pivot, and can be easily found on Wikipedia, etc. or calculated. In fact, is also readily available, but whatever.
Going through the calculation gives us
2. Center of Mass Velocity
If you can’t use the parallel axis theorem (rare), the other method would be to decompose the kinetic energy into two parts: translational kinetic energy of the center of mass plus rotational kinetic energy of the rod about the center of mass. This is the same process used when calculating the kinetic energy of a spinning and translating rod (common problem in basic mechanics), for example. Thus, we write
where is the same as above, and the velocity of the center of mass is just
or in other words, the angular velocity of the rod times the distance from the pivot to the center of mass. As expected, this gives the same result
To be honest, this inaugural post was written partly because I wanted to play with LaTeX/Inkscape and partly to get something onto my blog so it wouldn’t look this shabby anymore.