In a direct impact, loss in kinetic energy is given by △K=M1M22(M1+M2).(V1−V2)2(1−K2) with usual
notations(except k). The quality k will have dimensional formula:
Since 1 and k2 are being subtracted , they should have the same dimensions. 1 is a number i.e. dimensionless
Actually, k is the coefficient of restitution.