Block `A` is hanging from vertical spring of spring constant `K` and is rest. Block `B` strikes block `A` with velocity `v` and sticks to it. Then the value of `v` for which the spring just attains natural length is

Options

A. `sqrt(("60 m g"^(2))/(k))`
B. `sqrt(("6 m g"^(2))/(k))`
C. `sqrt(("10 m g"^(2))/(k))`
D. none of these

Answer

B

Explanation

Since water rises to height of 2 cm in a capillary

If tube is at `60^(@)`, in this case height must be equal to
`h = 2 cm rArr cos 60^(@) = (h)/(l)`
`:. lamda = (h)/(cos 60^(@))=(2)/(1//2)=4 cm`