- #1

- 349

- 15

## Homework Statement

The luminosity of the star is defined as ##\phi = L/4\pi r^2##

The photons from the star hits the sail of mass ##M## and cross sectional area ##A##. What's the final velocity of the sail after getting hit by photons for a small time t? Assume the collision is elastic and the photons reflects after hitting the sail.

## The Attempt at a Solution

##\delta p_s = 2\delta p_p= 2\frac{E}{c}=2\frac{LAt}{4\pi r^2c}##

##M\delta v = 2\frac{LAt}{4\pi r^2c}##

##v_f = 2\frac{LAt}{4M\pi r^2c} +v_i##

Is this correct? Because I assume the photons bounce back with a momentum of magnitude equal to initial but that's not stated anywhere.