In a cylindrical vortex motion about a vertical axis, r1 and r2 are the radial distances of two points on the horizontal plane (r2 > r1). If for a given tangential fluid velocity at r1, the pressure difference
between the points in free vortex is one half of that when the vortex is a forced one, then what is the value of the ratio (r2/r1)?
a. 
$\sqrt{\frac{3}{2}}$ b. $\sqrt{2}$
c. 3/2 d.$\sqrt{3}$