Week 9: That darn dent

I don't-slash-can't drive yet.

And I'm okay with that--I'm in no rush to get behind the wheel. Because I know that once I do, I would have to worry about drivers who do not stop to think about others. *cough*jerks*cough*

Exhibit A here serves two purposes for this blog:

1) It supports the view that I have stated above. 2) It shows us an example of momentum and its conservation in a collision.


Exhibit A



This dent, shown here, belongs to my father's Honda Accord. Fortunately, my father wasn't anywhere near his car at the time of collision. He parked the car at...um, I don't really remember where he was when this happened, so let's pick a place that men usually frequent...Home Depot? All right, so my dad parked his car at Home Depot, got out and went inside the store. After half an hour, he returned (carrying whatever he went in to buy), only to find that someone *cough*jerk*cough* had dented his car.

Momentum is the product of an object's mass and velocity. When two objects collide, the total momentum of the system is conserved.

My dad's car, with mass 907 kg, was stationary so its initial velocity was 0 m/s. Therefore, it had zero momentum. The inconsiderate driver may have been driving at 10 mph (4.4704 m/s--Is this the average speed limit in a parking lot? If it is, good. If not, let's just pretend that it is.) with a total mass of 1800 kg. After the collision, the two cars "join together" and move at the same speed So...how fast were the two cars moving after the collision?

My dad's Accord and the cuplrit's car were moving at 2.97 m/s. (-->)

This is an example of an inelastic collision because, while momentum was conserved, total kinetic energy was not. The total kinetic energy of the system decreased after the collision and the lost energy was probably converted to others forms--sound (I'm pretty sure that the collision caused some form of noise) and heat.

The dent on the car also shows the concept of impulse (J). Impulse, the change in momentum, is the product of force and time of the impact or collision. Impulse is constant because momentum is conserved. This means that if t increases, then F decreases. And if F increases, t decreases.

The dent in the car shows that the material "gave" or allowed for t to increase in order to lessen the force of the impact. If the car was made of a harder substance that does not easily dent, then the magnitude of the force would have been greater.

And that means more damage and more money. But that wouldn't be such a bad thing if, you know, all drivers were considerate. My dad wouldn't have had to pay for the repair all by himself (with the help of insurance, of course) if the inconsiderate driver had at least left him a note with an apology, and his/her name and number.

I mean, come on. If you know that you dented someone's car, how can you just drive away as if you didn't? That does not make any sense. And it probably never will make sense to me, even when I do start driving. I just know that I wouldn't be so inconsiderate.

And, voila, the Physics of that darn dent.