[Maxatar]

Technically the formula for force is F = dp/dt, or the derivative of momentum with respect to time.

For particles with mass the momentum of such a particle is p = mv, and so you can use that to yield F = ma. However for a massless particle like a photon, its momentum is p = E / c. If you use that momentum to describe a beam of photons being absorbed by a material, then you get F = n * E / c, where E is the average energy of the photons, n is the rate of photons per second, and c is the speed of light.

[mech422]

Oh Thanks! Should have known I was missing something basic :-)

edit: btw - thats a REALLY nice explanation :-)

Thanks