" ball of mass 100gms and another ball of mass 120gms moves towards each other with speed 6m/s and 5m/s ." So we will assume that the velocities are of respective balls.
Conservation of linear momentum,
Since no external force is acting, the linear momentum before and after the collision are equal.
And since the two balls move towards each other, means their velocities are oppositely directed, hence a negative sign for any one velocity.
Linear momentum before collision = Linear momentum after collision
(0.1 kg)(6 m/s) + (0.12 kg)(-5 m/s) = (0.22 kg)(v m/s)
0.6 kg.m/s - 0.6 kg.m/s = (0.22 kg)(v m/s)
v = 0.
The balls will come to rest.
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