A cart of mass $M$ enters a smooth vertical loop the loop or radius $R$ with a horizontal speed $u$ at the bottom of the track, as shown. The cart just manages to stay in contact with the track at the top most point of its path.
Its speed at the bottom of the loop $u$, is given by:
A. $u=\sqrt{5gR}$
B. $u=\sqrt{4gR}$
C. $u=\sqrt{gR}$
D. $u=\sqrt{2gR}$
