Cable that holds the elevator got disconnected

[Cliff_J]

Originally posted by Gara
and yes, jumping up at 120 mph would stop you from dieing from the fall of the lift, but presents a new, head smoosh style, problem.

unless you had exceptional timing! 7..6..5..4..3..2..ONE!

Cliff

 

[Gara]

if you jumped up a lift at 120mph, you have to remember the roof of the lift is about 7 foot high. unless you can slow down after the very moment you jump, you're going to hit the roof :)

 

[Cliff_J]

But if you jumped up so that you reached 120MPH relative to the elevator right at the moment of impact, your velocity would be zero relative to the ground.

Besides the roof maybe hitting you as it decelerates (assuming it decelerates differently from the floor as the elevator buckles from the impact) you would be, in effect, simply standing on a crumbling floor. And if you have the timing plus incredible strength (and luck) like that, you should run out and buy a lottery ticket, maybe just after kissing the ground and saying a couple prayers...

Cliff

 

[1234]

then what about if you weren't in the elevator but on top of it. what happens if you jump off it just before it hits the ground? would you survive?

 

[Integral]

One more time.

Jumping will not save you. If you could jump in a free fall (it is not clear to me how you would do this!)you will not change your final velocity. In fact, if the elevator where being slowed by some mechanism which would then enable you to jump, you would hit the ground at a HIGHER speed then if you had stayed with the elevator. This because during the time of your jump the elevator will have fallen some distance, you will have to fall that extra distance and will have a longer time of acceleration, thus a HIGHER VELOCITY when you finally hit the ground. How will this help you?

Consider this, if the elevator is going down at its normal rate you can jump up and down as much as you like, each time you will land back on the elevator, since the elevator is moving you will have to be falling FASTER then the elevator in order to catch up to it after you jump. This will be true even if the brakes fail and elevator is accelerating at some rate less then g.

So in summary now only will jumping not help it will only serve to INCREASE the speed at which you hit bottom.

 

[Gara]

i don't think so. if the lift is falling at 30 mph, and you jump up at a speed so you are now at 0 mph, you will hang there and then gravity will take over again, and you will fall, but for only the distance between you and the floor, which if timed right, should be about 1 foot.

but i must say again, you have a high chance of living by staying in the lift because at the bottom of lift shafts are springs and dampers to make for a soft landing in the event of a lift cable failing.

 

[Michael D. Sewell]

Originally posted by Gara
i don't think so.

Integral,
Some peoples haids is hardern chinese arithmetic. Godspeed, good luck, and god bless.
-Mike

P.S. Run away! Run away!

 

[Integral]

i don't think so. if the lift is falling at 30 mph, and you jump up at a speed so you are now at 0 mph, you will hang there and then gravity will take over again, and you will fall, but for only the distance between you and the floor, which if timed right, should be about 1 foot.

Gara, Gara, Gara...

It is not your speed relative to the elevator which is the problem. The only speed that matters is your speed with respect to the ground. What looks like a jump inside the elevator is not seen as a jump from the ground outside the elevator. If the passengers speed with respect to the elevator were of significance then why jump, you are not falling at all, but standing very peacefully on the elevator floor.

 

[UltraPi1]

I do believe Integral is wrong here. Assuming you had a super human pair of legs - You could time a jump with the proper force to where it would seem as if nothing happened. Thats of course if the elevator doesn't bounce on impact.

The elevator will eventually attain terminal velocity due to air pressure and whatever drag it may encounter. You on the other hand are not subject to these effects, and will fall at the rate specified by gravity alone. You could have a scale in the elevator and find that you weigh at least something - be it one pound, or maybe even ten. Whatever the number - You can push off of the elevator. It's just a matter of transferring the energy from you to the elevator. The elevator will get a sudden boost in speed, and if timed right with the proper force - you will only hear a very loud noise.

 

[Integral]

Assuming you had a super human pair of legs

And if pigs had wings they could fly.

Even with a super human (how stupid can it get!)legs, you will not change the speed at the time of impact. As I said in a previous post (Did you bother reading them?) if the elevator is not in a free fall situation, a jump, ANY JUMP will only serve to increase your speed at impact. There is NO way that you will hit any slower then the speed of the elevator. Remember that old rule, any thing which goes up must come down. You can jump but you have to land.

 

[UltraPi1]

Integral
You are flat out without a doubt ---- wrong!

I suggest you start reading some books, and come back when you got it figured out, so you can say you had it flat out without a doubt ---- wrong!.

Next thing yer going to tell me is that if I fired a bullet straight up in a falling elevator - It would never get out of the barrel. Your fantasy world does not exist.

 

[Evo]

Originally posted by Integral
And if pigs had wings they could fly.

Which means they might survive the elevator crash unharmed as opposed to the jumping humans.

 

[UltraPi1]

Which means they might survive the elevator crash unharmed as opposed to the jumping humans.

A flying pig would have to flap those wings double time at ground zero, but yeah - A flying pig could pull it off too if their wings could handle the stress. How much stress can a flying pigs wings handle anyway?

 

[Integral]

Originally posted by UltraPi1
Integral
You are flat out without a doubt ---- wrong!

I suggest you start reading some books, and come back when you got it figured out, so you can say you had it flat out without a doubt ---- wrong!.

Next thing yer going to tell me is that if I fired a bullet straight up in a falling elevator - It would never get out of the barrel. Your fantasy world does not exist.

Physics please?

Can you keep it close to the topic?

Back up your words with physics or do not post. Further off topic noise will simply be deleted.

 

[BasketDaN]

Originally posted by Integral
Gara, Gara, Gara...

It is not your speed relative to the elevator which is the problem. The only speed that matters is your speed with respect to the ground. What looks like a jump inside the elevator is not seen as a jump from the ground outside the elevator. If the passengers speed with respect to the elevator were of significance then why jump, you are not falling at all, but standing very peacefully on the elevator floor.

Even when your body is in the act of flying upwards while jumping inside of the falling elevator, this speed is simply subtracted from the speed that the elevator is falilng,, so you would ultimately still be falling down very fast.

 

[UltraPi1]

Even when your body is in the act of flying upwards while jumping inside of the falling elevator, this speed is simply subtracted from the speed that the elevator is falilng,, so you would ultimately still be falling down very fast.

This what I was trying to get across to Integral. I said if you were super human - you could negate that speed entirely. He seems to think otherwise. In fact he insist you would hit ground zero faster than the elevator.
In the real world - you would be in deep dodo ... no question about it. The original question of the thread was whether jumping could make a difference.
Under the right conditions - In the real world - It could.

 

[Integral]

Mr. Ultra pi,all leg muscle and no brain, at least after you mash your head into the ceiling of a falling elevator. Yes, if you could jump with a velocity equal to that of the falling elevator there would be a point at which you would have zero velocity with the outside world, unfortunatly that would probably mean that you will meet the ceiling of elevator with a relative velocity twice that (very nearly at least) at which the elevator is traveling. I thought the point of the question was can you save yourself by jumping. The answer remains no, you have simply added another way of dying.

 

[Gara]

that's what I've been saying! IF you had super strenth you could do it.

and i acually think you won't hit your head, if you jump up at the same speed the lift is going down, asuming as you hit the apex, (0 mph) the lift hits the bottom and stops moving, then you just free fall the tiny distance to the floor.

i suggest that since this is all about IF, it should be moved to the thory section.

 

[UltraPi1]

Mr. Ultra pi,all leg muscle and no brain, at least after you mash your head into the ceiling of a falling elevator.

A properly timed jump with the appropriate force, and you would not hit the ceiling. In fact you wouldn't have to leave the floor. All you have to do is equal the speed of the falling elevator, thereby canceling out. This is all under optimal conditions mind you. Wrong again.

I thought the point of the question was can you save yourself by jumping.

And I have said - Under the right conditions - it could make the difference ... however small it is. Even without super human legs. I said at the beginning of this thread ... Parameters would have to be set to answer that question. How big is the elevator? How much does it weigh? How well sealed is the shaft? How much space is there between the elevator exterior to the shaft walls?

 

[Integral]

A properly timed jump with the appropriate force, and you would not hit the ceiling. In fact you wouldn't have to leave the floor. All you have to do is equal the speed of the falling elevator, thereby canceling out. This is all under optimal conditions mind you. Wrong again

This is getting to be pure nonsense, where do you get this stuff from?

If your feet never leave the floor of the elevator then you are traveling the same speed as the elevator .. going DOWN! If you wish to cancel the speed of the elevator you must jump UP, now the only thing you have to push against is the FLOOR of the elevator. If you apply a force to the FLOOR you will accelerate with respect to that floor. So applying a force to the floor and staying at rest wrt that same floor is nonsense.

What is important here is your speed wrt to the ground, if you can hit the ground with speed approximately 0 then you will be fine. This means you must have a velocity wrt the elevator of V up.

Now a good question is what is a fatal velocity, is a 20m drop fatal? Maybe not, but I would not want to try it. A bit of computation shows that

$$v = \sqrt {2gx}$$
This gives a value of ~20m/s for a 20m fall. It is humanly impossible to cancel a fall at this rate. Unless you are a pretty good athlete a standing jump of .5m is pretty good, to jump .5m you need an initial velocity of about 2.25m/s.

Needless to say even for 10m/s fall a perfectly timed jump up at 2.25m/s will not help.

 

[UltraPi1]

This is getting to be pure nonsense, where do you get this stuff from?

It would seem you're going to fight this one all the way to the ground. Which is about where you got to make yer jump. I might as well call you pig headed .. with wings of course. You would hit the ceiling if you didn't time your jump. If the elevator were dropping at 70MPH and you made your jump 30 feet before you hit ground, and you were capable of jumping fast enough and hard enough to cancel the fall. You would hit the ceiling in the neighborhood of 70 MPH - (propably faster).

You would likely make your jump in the vicinity of 3 feet off the ground. Using your super human legs mind you. At the same instant you finish with the appropriate jump - The elevator hits ground. All things perfect - All things canceled. It would be as if nothing happened. Accept for the very loud boom at ground zero. Since the elevator is no longer dropping - Theres no need to worry about the ceiling because you are dropping at 0 MPH in relation to the ground due to your canceling jump, and the elevator is dropping at 0 MPH because it hit ground.

Now a good question is what is a fatal velocity, is a 20m drop fatal? Maybe not, but I would not want to try it. A bit of computation shows that

Yes ... that would be a good question regarding a personal drop in a free falling elevator. The most important parameters to consider would be the weight of the elevator, the surface area of the floor, how much space is there between the outside walls of the elevator and the shaft walls, is the shaft sealed well .. below the elevator? I would get in an elevator that had one eighth inch clearance between the exterior walls of an elevator and the shaft walls --- in a free fall. Providing there was no escape for the air below the elevator other than the one eighth clearance. It could very well be used as a ride at an amusement park.

Now to the real world elevator. How fast will it drop? This can't be set without setting parameters for it. I'm sure there are some elevators out there where if all the safety installs failed. You could expect a decrease in velocity toward the end of your ride before you hit ground, and there are some elevators out there where you could kiss your ass goodbye in a complete system failure. Now should you ever run into this kind of situation - I say jump ... Go ahead and jump, because you aint got time for love in an elevator. :-)

 

[outandbeyond2004]

I assume someone is atop on a block of metal that is freefalling towards an airless planet the mass of Earth. The block and the person has masses $$m_B$$ and $$m_P$$, which are negligible compared to the mass of the planet. So, from

conservation of momentum

$$(m_P + m_B)v = m_Pv_P + m_Bv_B$$

and conservation of energy (potential energy can be ignored as this is instanteous change of energy)

$$(m_P + m_B)v^2 = m_Pv_P^2 + m_Bv_B^2$$

where $$v, v_P, v_B$$ are respectively the initial velocity, the velocity of the person at the moment of final velocity change caused by the jump, and the velocity of the block at the moment of final velocity change caused by the jump.

Solve for $$v_P$$:

$$v_P = \frac{(m_P + m_B)v^2 - m_Bv_B^2}{(m_P + m_B)v - m_Bv_B}$$

Let

$$v_B = \gamma v$$

and

$$\kappa = \frac{m_B}{m_P + m_B}\gamma$$

The equation for $$v_P$$ can be rewritten:

$$v_P = v\frac{1-\kappa\gamma}{1-\kappa}$$

It is now evident that by some judicious combination of values for $$\kappa$$ and $$\gamma$$ we can have any value for $$v_P$$ including escape velocity away from the planet.

 

[chroot]

Integral is fully correct. Even if you could jump, and even if there were no ceiling in the way, you would go up, and eventually land with the same velocity the elevator landed. All you could do is delay the impact slightly. You can't beat the conservation of energy.

- Warren

 

[Integral]

First of all, in a properly designed elevator, this is simply not a problem, so why are we even thinking about elevator design? Remember the original question, it concerns a falling elevator in which the passenger is in danger (I assume this 'cus otherwise there is no question).

Therefore we have an elevator falling at a dangerously high speed, particulars of the elevator are not important. What is this speed and how does it compare to the speeds which a jumping human can hope to generate? By my estimates "dangerously fast" will be anything from about 10m/s up. 10m/s sec is equivalent to a free fall drop of 5m (This is a survivable drop, but I personally would not want to jump from 5m onto concrete (water is ok!)as serious injury is possible (even in water if you hit wrong!).

Ok, now to completely cancel the elevators velocity a human would have to jump up wards at precisely the right time with a velocity equal and opposite to that of the elevator. How many people do you know that can do a standing vertical jump of 5m? It is virtually impossible to cancel the velocity of even a survivable fall rate.

Now on top of this comes an additional problem, when to jump, in order to cancel the elevators velocity you must have the maximum upward velocity at the instant the elevator hits the ground, now the million dollar question is... How do you know when the elevator is going to hit? This is information that is not available, so I am right back to my origin statement... Jumping will not help. Simply because it is not enough and impossible to time correctly.

 

[chroot]

And, to reiterate my point, even if you could jump up from the floor of the elevator at 10 m/s and thus avoid being killed by the fall, you would eventually land at 10 m/s anyway, and die just a short time later.

- Warren

 

[Integral]

A key point here is that when falling at a velocity V, your body had kinetic energy mV²/2 it does not matter whether this energy is dissipated by jumping or stopping the effects on your body will be the same.

 

[Janus]

One thing that seems to be missed with all this super-human legs business. Your body would still have to absorb the forces involved in coming to a stop. It would just be spread out over the distance over how much you can bend your legs.

For instance, assuming you are trying to cancel out a falling speed of 60mph (88 ft/sec) and you maximum leg flex is 2 ft (from crouch to extended), the rest of your body would have to be able to withstand 60g. (forget about what happens later, the jumping itself would kill you.)

To survive, you would need a combination of enough leg strength and g force resistance. But if you had these, then you don't need to jump in the first place. You can just allow yourself to go from standing up to a crouch as you hit and let your legs absorb enough of the deceleration(like a shock absorber) to let you come out unharmed.

 

[outandbeyond2004]

A post considered jumping hard enough to cancel the downward speed. That is not possible if my equations in a prior post are correct. Substitute zero for $$v_P$$ and solve for $$v_B$$. The result is the same as the downward speed just prior to the jump. This is possible only if the mass of the person is zero.

You can survive any value of personal KE as long as you have time to dissipate it. However, a jump certainly does not give you much time, and if you have to jump to dissipate mucho KE, that would HAVE to be a very, very brief jump. It is said that people can survive 20 Gs, but I rather doubt even Olympic-class athletes can match that by jumping. Barry Sanders, the football carrier -- nah.

So I agree that jumping will do little good, and maybe it would be better to get to the top of the elevator, if at all possible, in the hope that the elevator can cushion the impact well enough.

A racecar driver survived a 170mph crash, because the "cell" he was inside absorbed enough energy slowly enough to let him survive.

 

[UltraPi1]

Integral is fully correct. Even if you could jump, and even if there were no ceiling in the way, you would go up, and eventually land with the same velocity the elevator landed. All you could do is delay the impact slightly. You can't beat the conservation of energy.

Wrong also. When you jump - You are just transferring that potential energy from you to the elevator through the use of another energy source, and that would come from those superhuman legs.

 

[Integral]

Wrong also. When you jump - You are just transferring that potential energy from you to the elevator.

What potential energy? Potential energy is not a problem,the problem is kinetic energy.
Can you explain the difference between the elevator suddenly stopping and you jumping? If you change your velocity from -V to 0 the change in velocity is the same, Remember the old saying "Its not the fall that hurts, its the sudden stop at the end". It does not matter what causes the sudden stop the forces experienced by your body will be identical in either case.

You need to read some of the other posts.

 

[UltraPi1]

One thing that seems to be missed with all this super-human legs business. Your body would still have to absorb the forces involved in coming to a stop. It would just be spread out over the distance over how much you can bend your legs.

Not missed - Just not mentioned. Make no mistake about it. This is a job for a superhuman. But you seem to at least understand the process of being able to slow your descent to zero by taking the hit before ground zero. Some here do not.

 

[UltraPi1]

Can you explain the difference between the elevator suddenly stopping and you jumping? If you change your velocity from -V to 0 the change in velocity is the same, Remember the old saying "Its not the fall that hurts, its the sudden stop at the end". It does not matter what causes the sudden stop the forces experienced by your body will be identical in either case.

You seem to be evading the original disagreement from the content of your last few post. I'll take that as a ... yes .. I was wrong, and accept that you just don't want to say so. At any rate I'm done here.

 

[Integral]

slow your descent to zero. Some here do not.

How do you SLOW you decent. You have only mentioned how to change it SUDDENLY, it is the sudden part that causes problems.

 

[Michael D. Sewell]

Originally posted by Integral
By my estimates "dangerously fast" will be anything from about 10m/s up. 10m/s sec is equivalent to a free fall drop of 5m (This is a survivable drop...

Integral,
This is a very good estimate. You are even being a little generous. In skydiving we consider 3m/sec to be a safe rate of descent for landing; higher speeds are OK with proper technique if the jumper is in excellent physical condition.

In construction, falling two stories is considered to be survivable. Falls involving distances greater than that rapidly decrease your chances for survival. This is a well documented fact. -Mike

 

[Integral]

Michel,

Thanks for the infromation. I spent a bit of time searching for real data with no luck, finally just aked myself how far would I want to jump. In my youth I did a fair amount of jumping off rocks into water, I know that anything over about 5m takes courage (or lack of brains!).

I find it interesting that your safe landing speed is very close to the landing (and launch) velocity of a good jump.