A collision is a phenomenon that can be described in three distinct steps.
1. Firstly the two objects that would collide must approach each other. The two objects may be travelling towards each other as shown below in fig 1. They may also be moving in the same direction as in Fig 2 but if the first one is slower than the second one they would meet at a certain time.
Fig 1 Two objects moving towards each other directly
Fig 2 Two objects moving in the same direction but about moving closer to each other.
2. The two objects that are travelling towards each other must meet. That is they make contact. During this contact they interact by exerting forces on each other. The forces that they exerts on each other must be equal to each other according to Newton’s third law of motion.
Fig 3 The two objects make contact and exert forces on each other
3. The two objects then separate and move away from each other in opposite as shown in fig 4 or moves in the same direction but with increasing distance of separation as shown in fig 5.
Fig 4 The two objects move away from each other
Fig 5 Two objects move in the same direction but the distance between them increases with time