A body is said to be in motion when its position with respect to an observer changes with time. Because nothing in universe is in state of absolute rest therefore motion of a body with respect to any observer is called relative motion with respect to that observer. Relative velocity is the velocity of the body with respect to the observer and it is calculated by the formula

where = velocity of body

= velocity of observer

Let two bodies A and B moving with initial velocities m/s and m/s and their accelerations are m/s 2 and m/s 2 . If both bodies are at origin initially then,

(i) The time after which becomes zero is–