In the summer, one of the best things to do is to go to an amusement park.  For me, I cannot handle too many crazy, twisting roller coasters, but I love bumper cars.  And, as it turns out, bumper cars have many applications of both momentum and dynamics in the way they function and how they impact the riders.  For instance, when a bumper car and its passengers bump into another bumper car and its passengers, both cars and their passengers experience a change in direction; usually, if the car was originally traveling forwards, it ends up traveling in the backward direction after the collision.  Why?  Well, it all lies in the idea of conservation of momentum, that the momentum before equals the momentum after.  If one considers the two cars as a system, their momentum before is the sum of the mass of each car multiplied by its velocity.  This momentum is some number, and the momentum after the collision must equal this number.  So, the cars must "rebound" off of each other in the opposite direction of which they were traveling in order to fulfill the law of conservation of momentum.  But one must also consider the passengers who are driving the cars.  One of the funnest parts about the bumper cars is the kind of jolt the passengers feel when they crash into another car.  So, why do riders feel like they are going to head bang the steering wheel?  The answer is inertia.  Inertia is the idea that objects resist a change in motion.  So, when the collision between the bumper cars causes a change in direction of the bumper cars (conservation of momentum), the riders launch forward because that is the direction in which they were traveling.  Because humans have significant mass and therefore inertia (mass is directly proportional to inertia), the bodies resist the change in motion of the bumper car by continuing in the direction they were originally traveling in.  Luckily, a seat belt or lap bar stops riders from flying out of the car, and the bumper car madness continues.