I am trying to understand the effect that eliminating the mass m1 (m1 = 0) has on the active forces (Fa). I have gone through a scenario where m1 is taken into consideration (refer to uploaded images). The active force on m1 is from the spring and does not have m1 in the expression (Fs1 = (l-r)k), so I did not think that there would be any change. However the set of D'alemberts reduces if m1 is eliminated, so how does active force from the spring effect the remaining equation of motion. If more info or clearing up is required please do not hesitate to ask. Any help would be much appreciated. Cheers.