Electromagnetic waves?

interplanetary space contains many small particles referred to as interplanetary dust. Radiation pressure from the sun sets a lower limit on the size of such dust particles. To see the origin of this limit, consider a spherical dust particle of radius R and mass density p. a)write an expression for the... show more interplanetary space contains many small particles referred to as interplanetary dust. Radiation pressure from the sun sets a lower limit on the size of such dust particles. To see the origin of this limit, consider a spherical dust particle of radius R and mass density p.

a)write an expression for the gravitional force exerted on this particle by the sun(mass M) when the particle is a distance r from the sun.
b)let L represent the luminosity of the sun, equal to the rate at which it emits energy in electromagnetic radiation.Find the force exerted on the (totally absorbing) particle due to solar radiation pressure.

I can do part a but i having trouble with part b.
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