The Physics of a Dented Car

Sometime last year, my family and I went to Ala Moana to do some Christmas shopping. After 5 grueling hours of shopping, we got to the parking lot and found our car was sporting a huge dent in the back and found no note of explanation on the windshield. The lot next to our car was empty and we assumed that the driver of the "missing" car "accidentally" hit our car while backing out of the parking lot. Putting the missing identity of the car that dented our car aside, I realized that this connects perfectly with the concepts we just learned in Chapter 8.

The "collision" between car A and Car B (our car) illustrates an inelastic collision. Car A moves at velocity v with momentum p and kinetic energy k while Car B remains stationary with velocity 0, momentum 0, and kinetic energy 0 (parked). After the collision, kinetic energy was lost, as illustrated by the dent. An inelastic collision is when the total amount of kinetic energy in the system is reduced by the collision. As demonstrated by the dent, some of the kinetic energy was transformed by the collision into other types of energy, which decreases the total kinetic energy. The dent shows that some kinetic energy was used to compress the car and that some kinetic energy was also lost to thermal and sound energy (bang after the collision).

Furthermore, the collision also illustrates Newton's third law of motion, which states that there is an equal and opposite reaction to every action. Car A exerts a force on the stationary Car B and Car B also exerts a force that is equal and opposite to the force exerted by Car A on Car B. In addition, although kinetic energy is not conserved, the momentum is conserved because momentum is always conserved regardless of the nature of the collision (whether it is inelastic or elastic).