Real-Life Math -- Solution
1. The plane was at 5,000 feet AGL. You deployed your parachute at 2,500 feet AGL.
Distance fallen = 5,000 feet - 2,500 feet
Distance fallen = 2,500 feet
We know the first 1,500 feet lasts 12 seconds. The rest is at terminal velocity.
Remainder = 2,500 feet - 1,500 feet
Remainder = 1,000 feet
You fell 1,000 feet at terminal velocity. You know you fall 1,000 feet every 5 seconds at terminal velocity, so you fell for another 5 seconds.
Total time = 12 seconds + 5 seconds
Total time = 17 seconds
Your free-fall time was about 17 seconds.
2. The plane was at 6,300 feet AGL. You deployed your parachute at 2,600 feet AGL.
Distance fallen = 6,300 feet - 2,600 feet
Distance fallen = 3,700 feet
The first 1,500 feet took 12 seconds.
Remainder = 3,700 - 1,500
Remainder = 2,200 feet
You fell 2,200 feet at terminal velocity. At that speed, you are falling 1,000 feet every 5 seconds.
Speed = 1,000 feet / 5 seconds
Speed = 200 feet per second
Fall = 2,200 feet / 200 feet per second
Fall = 11 seconds
You fell 11 seconds at terminal velocity.
Total time = 11 + 12
Total time = 23 seconds
Your free-fall time was about 23 seconds.
3. The plane was at 7,200 feet AGL. You deployed your parachute at 2,700 feet AGL.
Distance fallen = 7,200 - 2,700
Distance fallen = 4,500
The first 1,500 feet took 12 seconds.
Remainder = 4,500 - 1,500
Remainder = 3,000
You fell 3,000 feet at terminal velocity, which is about 200 feet per second.
Fall = 3,000 feet / 200 feet per second
Fall = 15 seconds
You fell 15 seconds at terminal velocity.
Total time = 15 + 12
Total time = 27 seconds
Your free-fall time was about 27 seconds.
Skydiving instructors calculate their free-fall time every time they jump. They also show students how to do it. Students receive instruction manuals with tables for calculating free-fall time.
"Free-fall math is about the only mathematics involved," skydiving instructor Sheldon Rice says. "Every single jumper keeps track of how many seconds of free fall they have."
In a logbook, skydivers keep a running total of their free-fall time. Just like pilots, they need certain amounts of time in the air before they're allowed to perform advanced techniques or get certified to teach.