And perhaps also a last comment about rotational kinetic energy and
internal energy. In which of the following cases would you say the
rotational energy is internal? (I would say all of them.)
1. A rigid rotating disk.
2. A system of two identical rigid disks rotating with opposite
angular velocities (so that the angular momentum of the system is
zero).
3. The wheels on a car driving down the street. (System = whole car.)
4. All the rotating axles, bearings, motor shafts, and other gizmos
under the hood of that car.
5. The rotating gas molecules in the engine of that car.
When a cylinder rolls without slipping down an incline, it loses
mechanical energy (grav. PE + transl. KE of COM) and gains internal
energy (rot. KE). When a person accelerates up to walking speed, she
gains mechanical energy (transl. KE of COM) and loses internal energy
(biochemical energy + thermal energy + KE rel. to COM). In both
cases, the coupling between the two forms of energy is mediated by
static friction (I'm neglecting air resistance, heat transfer to
surroundings, and contact deformations such as "rolling friction") --
turn off static friction and the cylinder/person can't roll/walk. Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040 mailto:mungan@usna.eduhttp://usna.edu/Users/physics/mungan/