From the principle of conservation of momentum; it implies momentum before collision is same after collision hence
M1U1 +M2U2= M1V1 +M2V1
Since both objects comes to rest after collision it means the final velocity is 0 and this has been represented as V1; hence the right side of the equation evaluates to zero.
We can then rewrite the equation as;M1U1 +M2U2=0
Let U1 and U2 be the respective velocity for M1 =30kg and M2= 50kg respectively.
Hence the equation becomes on substituting M1,M2 and U1.
We have;
30×5 + 50×U2 = 0
150 +50U2=0
50U2 = -150
U2= -150/50 = -30m/s
Ignore the -ve sign which shows the direction of the 50kg mass was opposite that of 30kg
