We distinguish between two types of collisions: (a) linear (b) oblique. The collision is linear if at the moment of collision the line joining the centres of the two colliding spheres lies in the same direction as that of both the velocities before collision. The collision is oblique, if this condition is not satisfied.
Let us consider two spheres designated 1 and 2 of equal mass m and of equal radius (Fig. 4.5). The spheres are supposed to be perfectly smooth. The mechanical laws obeyed by the colliding spheres
Fig. 4.5 Illustration of linear collision