[soupastupid]

Suppose that you designed a spacecraft to work by photon pressure. The sail was a
completely absorbing fabric of area 1.0 km2 and you directed a red 1000 Watt laser
beam of wavelength 650-nm onto it from a base on the Moon.
a) The number of photons per second striking the sail?
b) What is the total force on the sail in one second?
c) What is the pressure exerted by the radiation on the sail?
d) Suppose the mass of the spacecraft was 1.0 kg. After a period of acceleration
from standstill, (speed = force/mass × time) how long would it take for the
craft to accelerate to a speed of 1.0 m s-1?

For A)
i used
vf = c /lambda
where vf is frequency
to solve for frequency

vf= (3.0*10^8 m/s) / (650 * 10^-9 m ) = 4.6*10^14 1/s
this is the number that passes by the observer every second?
do i need this number?

then i used E = hc/lambda
to solve for energy of a photon
E = (6.626*10^34 J-S)(3.0*10^8 m/s) / (650*10^-9 m) = 3.06*10^-19 J

then I used the power to solve for the amount of photons?

1000 watts = 1000J per second
so if i divide 3.06*10^-19J / 1000J
3.27*10^21 photons per second

B) Force
Force = ?? i cant use ma
photon has no mass

[Borek]

Force is dp/dt.

[Juan R.]

photon has no mass

It is believed that Photons have zero rest mass.

http://www.phys.ncku.edu.tw/mirrors/physicsfaq/ParticleAndNuclear/photon_mass.html

Also I would also add that dp/dt is related to force F and not to force Q.