4. In a physics experiment, two balls \(A\) and \(B\), of mass \(4 m\) and \(3 m\) respectively, are travelling towards one another on a straight horizontal track. Both balls are travelling with speed \(2 \mathrm {~ms} ^ { - 1 }\) immediately before they collide. As a result of the impact, \(A\) is brought to rest and the direction of motion of \(B\) is reversed.
Modelling the track as smooth and the balls as particles,

1. find the speed of \(B\) immediately after the collision. A student notices that after the collision, \(B\) comes to rest 0.2 m from \(A\).
2. Show that the coefficient of friction between \(B\) and the track is 0.113 , correct to 3 decimal places.