2 Two toy trains, \(A\) and \(B\), are moving in the same direction on a straight horizontal track when they collide. As they collide, the speed of \(A\) is \(4 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) and the speed of \(B\) is \(3 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). Immediately after the collision, they move together with a speed of \(3.8 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). The mass of \(A\) is 2 kg . Find the mass of \(B\).