Calculating Photon Momentum & Force

[ride4life]

Homework Statement

a) Determine the momentum of a photon of light of wavelength 400nm.
b) A mass of 1mg is being held up by a beam of 400nm light. How many photons per second would have to hit the mass (from below) to do this, assuming the photons bounce back with the same momentum as they hit.

Homework Equations

a) p=h/(wavelength)
b) F=mg

The Attempt at a Solution

a) p=6.64x10^-34/400x10^-9=1.66x10^27 kgm/s
b) F=10^-3x9.81=9.81x10^-3N
I can only figure out how much force is needed to keep the mass still, I don't see how p and F are related :(

 

[Jolsa]

Try using Newtons second law in its "original" form.

 

[cupid.callin]

yes i think it will solve the problem!

Its F = dP/dt

 

[sigmaro]

Don't be confused, i am sure you know the formula cupid.callin mentioned, F = dP/dt,
the problem asks you to find photons per second so you found one of the photons momentum, you try to reach dP/dt so you have to product p with dn/dt what is asked, then reach dP/dt which is force
consider P is the total momentum and p is the momentum of one photon