Sentence:  Further , the corpuscular radiation , i.e. , the solar wind protons , must sputter away the surface atoms of the dust and cause a slow diminution in size , with a resultant increase in both the Poynting-Robertson effect and the ratio of the repulsive force to the <WSD>gravitational</WSD> force . 
Meaning of the word marked with <WSD> tags:  (physics) the force of attraction between all masses in the universe; especially the attraction of the earth's mass for bodies near its surface