# Force or dp/dt for a photon?

1. Jan 31, 2009

### vette982

If p=E/c for a photon, and I'm given a bunch of other equations such as E=hc/λ and p=h/λ and E=hf and more...

Note: h=planck's constant, f=frequency, c=speed of light, E=energy, p=momentum.

I'm wondering how to find dp/dt (=Force)?

Original question:
Photons carry momentum, hence they exert pressure on the surface they strike. In this question, we we'll computer the approximate pressure at a distance r exerted by radiation from a star, using the relativistic equation between energy, momentum, and mass. Pressure is force exerted per unit surface area. The force exerted per particle can be written dp/dt where p is the momentum of the particle (from F ~ v^2 and p ~ v). Derive an expression for pressure P at a radius r from the radiation originating isotropically from a point source (i.e. a star), in terms of the momentum of the photons and radius r. (This is most easily derived by imagining that the photons are striking a spherical shell of radius r centered on the star).

Last edited: Jan 31, 2009
2. Feb 3, 2009

### malawi_glenn

it depends on if the photon is absorbed or totally reflected.

Are you looking for radiation pressure? Use stefan boltzmans law, or rather, derive it.