A body of mass 5 kg makes an elastic collision with another body at rest and continues to move in the original direction after collision with a velocity to 110th  of its original velocity. Then the mass of the second body is

Mass of the first body m1=5kg , for elastic collision e = 1                 Suppose initially body m1 moves with velocity vAfter collision velocity becomes (u10) .Let after collision velocity of M block becomes (v2) .By conservation of momentumm1u1+m2u2=m1v+m2v2    Or           5u+M×0=5×u10+Mv2Or             5u=u2+Mv2      ……………(i)  Since, v1−v2=−e(u1−u2)           u10−v2=−1(μ)Or       u10+u=v2               11u10=v2                            …………(ii)Substituting value of v2  in Eq. (i) form Eq. (ii)We get                     5u=u2+M(11u10)    Or    5−12=M(1110)Or                 M=9×102×11         Or        M=4511                           = 4.09 kg