Skydriving

A reader writes "Autoweek has this article about a group of guys in Arizona who will drop your car out of a cargo jet for $15k. Parachutes for the car are optional. Their ultimate goal is to drop a Greyhound with forty people inside. More pictures here and here)"

Re:Yes, but can they aim?

[lommer]

If you read the article, you will find that they don't plan to parachute the bus. It will just fall and smash to pieces, after all of the skydivers have bailed out the doors/windows and used their chutes as normal.

As far as i'm concerned, that would would be dope!

Re:"Gaining speeds of up to 140mph"?

[AntiNorm]

Mass is irrelevant. If you drop a bowling ball and an apple off a building, they will hit ground at the same time.

This is assuming no air resistance. Air resistance makes a pretty big difference; without air resistance, the concept of terminal velocity would not exist. A freefalling object would just keep accelerating and accelerating until it hit the ground.

As a quick demonstration of this, compare the real-world terminal velocity of a human body in the belly-to-earth position -- 120 mph -- with what it would hit on a typical skydive from 12000 feet, minus the parachute pull. I'll use an arbitrary weight of 190 lbs for the purposes of these calculations.

Gravitational potential energy = mass * g * height
Kinetic energy = (1/2) * mass * velocity^2
Energy is conserved between GPE and KE considering that we are assuming no air resistance.

Setting (190/32.2) * 32.2 * 12000 equal to (1/2) * (190/32.2) * v^2 and solving for v yields an impact velocity of 878 feet per second, or 599 mph. This is a far cry from the real-world result of 120 mph, so you can see that friction caused by air resistance makes an enormous difference. As for mass not mattering, mass only doesn't matter if you don't take air resistance into consideration because, as I demonstrated above, the mass is on both sides of the equation so it cancels out. But if you consider air resistance [sum of energy before = sum of energy after plus energy lost to friction], mass no longer cancels out. Incidentally, yes, different skydivers (IAAS) do fall at somewhat different rates, but 120 mph is the generally accepted standard. Different people have different masses and different surface areas, so their terminal velocities will be slightly different.