Question

radiation force on cone due to parallel beam of light on perfectly absorbing surface

Answer 1

F = P/c

Answer 2

For a perfectly absorbing surface, all incident light momentum is transferred. The total momentum transferred per unit time (which is force) equals the total incident power divided by the speed of light ($F = P/c$). For a parallel beam incident on a cone, the total power intercepted ($P$) is the product of the beam's intensity ($I$) and the cone's base area ($\pi R^2$), so $P = I \pi R^2$. Thus, the radiation force is $F = \frac{I \pi R^2}{c}$.