In many problems, two objects are either approaching each other, chasing each other, or trying to get away from each other. Some examples might be: a police car chasing a speeding car, a passenger chasing a departing train or bus, an ambulance moving through traffic, two cars moving through an intersection, two vehicles coming towards each other on a twoline road, or two onedimensional projectiles traveling in the same or opposite directions while moving through the air.
s_{pursuer} = "gap" + s_{leader}

v_{o}t + ½at^{2} 
number

v_{o}t + ½at^{2} 
vt


vt

Each column in the above table states the allowed behaviors for the pursuer and the leader. Each participant can either be experiencing accelerated or linear motion. The numerical value of the "gap" can be equal to zero (if the two objects start sidebyside) or it can be a nonzero number. The parameter t, for time, unites the equations. To solve chase equations, you first determine the time that is required for the two objects to come together  then, you use that time to determine the position of their collision.
To work this type of problem, one object is considered the leader and the other is the pursuer. The pursuer, in reaching the leader's final location, must not only close the leader's original gap but also account for any subsequent displacement the leader travels while being chased. In the first example, 