# Why would I die when I jump out of a plane?

1. Mar 18, 2005

### theboeh

So if F=ma and the earth's gravity provides a constant acceleration then why would I die if I jump out of an airplane but not if i jump off a 10 foot ledge? I'm sorry if this is a stupid question but it's been several years since I was in a physics classroom.

2. Mar 18, 2005

### Pengwuino

Yes, its a constant acceleration which means your speed is increasing at a constant rate. A roughly 10m/s acceleration means that in 2 seconds, your traveling at 20m/s and at 10 seconds, your at 100m/second and if you live to 100 seconds, your traveling at 1km/second (well theoretically, friction slows you down).

3. Mar 18, 2005

### SpaceTiger

Staff Emeritus
It's also worth remembering that the force on your body has to do with how quickly you decelerate when you hit the ground, which is related to the velocity you're hitting it with. The reason that you're better off landing on a giant foam rubber pad than a pile of rocks is that the foam rubber pad will decelerate you more slowly (relatively speaking), inducing less of a force on your body.

4. Mar 18, 2005

### scilover89

The equation is:
Ft=m(v-u)
So, if you have higher velotion, you will receive greater force when you hit the floor

5. Mar 18, 2005

### whozum

The force you hit the ground with isnt what kills you, its the impulse, or time derivative of force. At 10m/s (25mph) you would survive a crash whereas at 100m/s (250mph) you are going to impart alot more energy in the same time interval (much higher impulse) which is what kills you.

Keeping air resistance in mind, terminal velocity is about 100-150 mph, so once you hit that speed your not going to accelerate any longer and will be in freefall. Given a height of jump "h1" where terminal velocity is reached during the fall, any height greater than h1 will result in the same speed of downfall and therefore the same impact velocity.

6. Mar 19, 2005

### SpaceTiger

Staff Emeritus
Impulse is

$$Fdt$$

not the time derivative of force. It represents the instantaneous momentum transfer, not energy.

Free fall is not when you hit terminal velocity. You're in free fall from the moment you jump.

7. Mar 19, 2005

### Andrew Mason

This is not correct. It is the force that kills you. If you landed on a huge air pillow (as stunt people do when they dive off buildings) you would not be hurt at all but you still experience the same change in momentum. If you increase the time over which the change in momentum occurs you reduce the force.

AM

8. Mar 19, 2005

### pervect

Staff Emeritus
The short answer is that you don't die when you jump out of an airplane, you only die when you hit the ground :-)

What's important is the velocity you have when you hit the ground. The velocity you have when you hit the ground depends both on the acceleration of earth's gravity and the distance you fall - v = sqrt(2*a*d) in the absence of air resistance, to be exact.

Last edited: Mar 20, 2005
9. Mar 19, 2005

### Chronos

I'm hesitant to leap off a 10 foot ledge and splat on concrete. I think it would hurt. But I don't doubt it would be fun to watch.

10. Mar 19, 2005

### tony873004

There are some people who have jumped out of an airplane and lived. I was watching something on TV a few years ago about survivors of parachute accidents.

That reminds me of...
"Guns don't kill people.... bullets do"
as a humorous rebuttal to the "Guns don't kill people, people kill people" bumper sticker.

11. Mar 20, 2005

### Danger

If the chute deployed without properly opening, it would act as an aerobrake to a far lesser extent than normally, but it would still decrease terminal velocity. Therefore, less splat. Also, the last case that I heard of landed on really soft ground.

12. Mar 20, 2005

### Staff: Mentor

What you just described is impulse (though I think it would be the time integral of force - force times time).

13. Mar 20, 2005

### Andrew Mason

Yes. Impulse = $\int Fdt = \Delta p$

AM

14. Mar 20, 2005

### Uno Lee

I fell down a cliff one time, I figure that it was about 10 m. In mid free fall, head first, I had the thought that I had better do something, so I somehow spun my body end for end , it must have looked like a back flip, and landed in a sitting position in a clump of vine maples with no impact. Does a spinning body land with less impact and if so why?

15. Mar 20, 2005

### whozum

This is exactly what I meant to say, I got the terms mixed up.

16. Mar 20, 2005

### whozum

I meant free fall in the sense of weightlessness, at terminal velocity his net force will be 0. Kind of like the astronauts in space.

17. Mar 20, 2005

### whozum

No, you may deflect the energy better by molding your body in a more comfortable falling position, but the same amount of energy is dissipated. It depends on how you spin.

But a spinning ball for example, spin will not affect the impact force much if at all.

18. Mar 20, 2005

### Andrew Mason

No. There may be a slightly slower impact speed for the part that is spinning 'up' when you hit but there is another part of you that will have a slightly higher impact speed (ie. the part opposite that is going 'down' when you hit). You can't slow down your centre of mass by spinning.

Now, in your fall, there could not have been 'no impact'. Fortunately you were not hurt. It seems that the branches that you landed on applied a gentle stopping force by bending and spreading that force out over some time and distance.

AM

19. Mar 20, 2005

### Creator

Yea, it's not the fall that gets you; its the sudden stop. he,he.

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--If at first you don't succeed, don't try skydiving! :yuck: --

20. Mar 21, 2005

### Uno Lee

Well, I did land with about as much impact as sitting down on a couch and I did have a minor scrape on my knee from the cliff but I was amazed looking up at the hight I had just fallen that nothing serious had happened.