A uniform rectangular plate is hanging vertically downward from a hinge that passes along its left edge. By blowing air at `12 m//s` over the top of the plate only, it is possible to keep the plate in a horizontal position, as illustrated in part (a) of the drawing. To what value of speed should the air be blown so that the plate is kept at a `30^(@)` angle with respect to the vertical, as in part (b) of the drawing ?

Options

A. `12 m//s`
B. `6 m//s`
C. `6sqrt2 m//s`
D. `12sqrt m//s`

Answer

C

Explanation

Taking torques about the hinge, we get:
In the `1st` case, `mgL//2 = p AL//2`
In the second case, `mgL//4 = p' AL//2`