Physics lol

A 200g torch is sitting in zero gravity.
It has a 20W bulb of perfect efficiency, which emits light of wavelength 120nm parallel to the torch.

Calculate its acceleration.

oh dear god

Cant be done. to hard.

Step 1:  How many photons are being shot off each second?
20W = 20 Joules / Second
E = hc/λ = 1.65*10^-18 Joules / Photon
20 Joules / Second divided by 1.65*10^-18 Joules / Photon
 = 1.21*10^19 Photons / Second

Step 2: What force do these photons generate?
Each photon has momentum of h/λ
h/λ = 5.52*10^-27 N*s / Photon
5.52*10^-27 N*s / Photon times 1.21*10^19 Photons / Second
 = 6.68*10^-8 N

Step 3: Calculate Acceleration.
6.68*10^-8 N = m*a
a = 6.68*10^-8 N / 200 g
a = 3.34*10^-7 m/s^2

wow.