# Photon Force

1. Aug 30, 2010

### ride4life

1. The problem statement, all variables and given/known data
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.

2. Relevant equations
a) p=h/(wavelength)
b) F=mg

3. 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 :(

2. Aug 30, 2010

### Jolsa

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

3. Dec 24, 2010

### cupid.callin

yes i think it will solve the problem!!!

Its F = dP/dt

4. Dec 25, 2010

### 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