What makes some collisions elastic and others inelastic?

Acar traveling at a constant speed of 22.0 m/s passes a trooper hidden behind a billboard. one second after the speeding car passes the billboard, the trooper sets off in chase with a constant acceleration of 3.50 m/s2. how long does it take the trooper to overtake the speeding car? solve this problem by a graphical method. on the same graph, plot position versus time for the car and the trooper.