Q. A string is wrapped around a disk of mass M and radius R. Starting from rest, you pull the string with a constant force F along a nearly frictionless surface. At the instant when the center of the disk has moved a distance d, a length L of string has unwound off the disk. At this instant, what is the speed of the center of mass of the disk?

Q. The temperature of the surface of a certain star is 7500 K. Most hydrogen atoms at the surface of the star are in the electronic ground state. What is the approximate fraction of the hydrogen atoms that are in the first excited state (and therefore could emit a photon)? The energy of the first excited state above the ground state is (-13.6/22 eV) - (-13.6 eV) = 10.2 eV = 1.632e-18 J.