First, moving objects have momentum (the product of the mass and the velocity of an object). If no outside force acts on an object, the object will continue to move at its present speed and direction (because of inertia). Cars consist of several objects, including the vehicle itself, loose objects in the car and, of course, passengers. If these objects are not restrained, they will continue moving at whatever speed the car is travelling at, even if the car is stopped by a collision. 