A roller coaster car $($mass $=$ M$)$ is on a track that forms a circular loop $($radius $=$ r$)$ in the vertical plane. If the car is to just maintain contact with the track at the top of the loop, what is the minimum value for its speed at that point?

Question $8$Select one:

a$.$

rg

b$.$

$($rg$)^1/2$

c$.$

$(0\mathrm{.}5$rg$)^1/2$

d$.$

$(2$rg$)^1/2$