Sentence:  On May 15 , a very large increase occurred with * * f of mass between * * f and * * f ; for the next two days , the <WSD>impact</WSD> rate was * * f ; and for the next nine days , the impact rate was less than * * f ( Nazarova , 1960 ) . 
Meaning of the word marked with <WSD> tags:  the striking of one body against another