A particle of charge $Q$ and mass $m$ travels through a potential difference $V$ from rest. The final momentum of the particle is:

In a crystal the atoms are located at the position of ________.       

A plank $P$ is placed on a solid cylinder $S$, which rolls on a horizontal surface. The two are of equal mass. There is no slipping at any of the surfaces in contact. The ratio of the kinetic energy of $P$ to the kinetic energy of $S$ is