Sentence:  The mass scale used in Table 5 - 1 was derived on the assumption that the <WSD>motion</WSD> of the glowing trail is related to the momentum transfer to the trail by the meteorite , permitting the calculation of the mass if the velocity is known ( Cook and Whipple , 1958 ) . 
Meaning of the word marked with <WSD> tags:  a natural event that involves a change in the position or location of something