02GF74
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 posted on 17/1/07 at 02:05 PM |
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physics questions - lifts
If one was to jump out of a high building onto a hard surface , one would go splat big time. This is because you have gained kinetic energy due to
gravity.
Now imagine this scenario. You are in a heavy lift and the cable snaps. As the lift falls downwards, half a second before it hits the ground you
jump up. When you push off the floor of the lift, it is heavy enough not to be pushed down, so you have decreased the speed at whcih you are
travelling. You have lost enough kinetic energy so you land gently on the lift floor.
Will you be able to push upwards hard enough to be able to survive?
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nick205
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| posted on 17/1/07 at 02:11 PM |
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Whether the "physics" says you can or not, my money is on you not surviving 
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smart51
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| posted on 17/1/07 at 02:13 PM |
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If the lift is traveling at 50 MPH when it hits and you have just jupmed up at 10 MPH then you "only" hit the ground at 40 MPH. This is
assuming the lift doesn't bounce at the bottom.
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bartonp
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| posted on 17/1/07 at 02:14 PM |
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quote:
Originally posted by 02GF74
If one was to jump out of a high building onto a hard surface , one would go splat big time. This is because you have gained kinetic energy due to
gravity.
Now imagine this scenario. You are in a heavy lift and the cable snaps. As the lift falls downwards, half a second before it hits the ground you
jump up. When you push off the floor of the lift, it is heavy enough not to be pushed down, so you have decreased the speed at whcih you are
travelling. You have lost enough kinetic energy so you land gently on the lift floor.
Will you be able to push upwards hard enough to be able to survive?
Only if you can jump up nearly as high as the lift came down from when the cable broke (ignoring windage).
I say nearly because obviously the human body can withstand some impact.
Phil.
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02GF74
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| posted on 17/1/07 at 02:16 PM |
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quote:
Originally posted by bartonp
Only if you can jump up nearly as high as the lift came down from when the cable broke (ignoring windage).
Phil.
that can't be right? surely it is to do with relative velocity - the speed you try to jump up at will be subtracted from the speed you andthe
lift are falling at?
[Edited on 17/1/07 by 02GF74]
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Tralfaz
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| posted on 17/1/07 at 02:19 PM |
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This was tested on a show called Mythbusters.
http://www.youtube.com/watch?v=3biD18ceNuE
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02GF74
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| posted on 17/1/07 at 02:23 PM |
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quote:
Originally posted by Tralfaz
This was tested on a show called Mythbusters.
.... and? (don't have access to youtube)
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iank
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| posted on 17/1/07 at 02:37 PM |
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Lift falling at 53 mph. crash test dummy jump was 2mph (and hit roof) and in many pieces.
Hit the ground at 50mph...
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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Colnago_Man
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| posted on 17/1/07 at 02:39 PM |
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You could also try stepping out the lift just before it hits the ground...I saw this on Tom and Jerry once, worked a treat.
You would need to jump up at the same speed the lift is falling, but as 'every action has an equal and opposite reaction' (quote I.Newton)
this may only increase the speed of the lift, eg you cannot win!
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IainB
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| posted on 17/1/07 at 02:56 PM |
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The way I understand it, your only hope is to increase the time taken to absorb the impact, ie from the time the lift makes contact with the ground
until your body comes to a standstill. A car crashing into a brick wall at 60mph is far more lethal than a car crashing into a pillow factory at
60mph! They both have the same kinetic energy but the 2nd car takes longer to absorb impact.
Not sure how you would apply this in a lift though! 
Iain
[Edited on 17/1/07 by IainB]
http://s43.photobucket.com/albums/e365/IainB1986/
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worX
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| posted on 17/1/07 at 03:03 PM |
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Bugs Bunny
Macnab 
I was about to quote the bugs bunny one, but felt it lacked relevance due to Bugs surviving purely becasue the plane ran out of fuel moments before
the impact and therefore stopped in midair (obviously) quality trick!!!
not sure about the lift and it running out of momentum though......
Steve.
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macnab
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| posted on 17/1/07 at 03:07 PM |
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Its works!! I saw it on the telly -
Bugs Bunny also saved himself from a plane crash by stepping out the door just as it smash into the ground...
He has actually been nearly killed in 2 plane acidents...
Lucky bunny.
Also apparently you only fall off a cliff when you notice your running in the air, as proved by wile Coyote…
[Edited on 17/1/07 by macnab]
 
Rescued attachment coyote.jpg
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Alan_Thomas
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| posted on 17/1/07 at 03:09 PM |
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Based on your scenerio next time I am on the point of driving into a stationary car at speed (never tried it, hopefully never will) all I have to do
is quickly slide the seat back on the runners.
- Alan
Blast! just remembered the Locost seat is fixed 
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Phil.J
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| posted on 17/1/07 at 03:11 PM |
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Fortunately lifts can't plummet anymore due to safety brakes, momentum activated. I think I,m right in saying there has only been one plummeting
lift accident since about 1930-odd, when a plane crashed into the Empire State building? I do remember as a kid in Lewis's department store in
Birmingham, the trellis gates of the lifts revealed a giant coil spring at the bottom of each lift shaft. Ever optimistic!
ATB
Phil
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macnab
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| posted on 17/1/07 at 03:12 PM |
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Rescued attachment CoyoteGenius.gif
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Donkeymatt
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| posted on 17/1/07 at 03:46 PM |
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As studied in year 1 Mechanical engineering in Cardiff University!!! If the lift drops at that velocity - you hit the roof at the same velocity, so
the chance of you "standing" on the floor is minimal - you will be pushed very hard against the ceiling, and then catapulted into the
floor at a very high speed when the lift stops! This usually causes a whole load of horrible death in both instances! Who says students don't
learn anything!!!
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02GF74
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| posted on 17/1/07 at 04:25 PM |
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I've been thinking more about this - donlt think its' gonna work becasue after you jumped up, you will come down due to gravity so you
speed of impace may be even higher.
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Johnmor
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| posted on 17/1/07 at 04:34 PM |
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"Pushed into the ceiling", surely not.
You will fall at the same speed as the lift and accelarate the the same speed as the lift. You will effectivly be weightless in relation to the lift
environment. Any attempt to "jump" will be pretty hard.
Try jumping in mid swimming pool.
If you were in contact with the lift floor and you tried to jump
half the enrgy would be transfered into downward movement of the lift.
You would bash you head on the roof. Not ideal.
Best hope is too attach your braces to the light fitting in the lift as it falls ( you will be weightless) and open your jacket as wide as poss. when
the lift hits the deck the combined wind resistance as you continue to fall and the extention of the elastic in the braces will result in a gentle
touchdown onto the lift floor.
Rlease braces, press door open button and exit.
Simple
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martyn_16v
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| posted on 17/1/07 at 08:41 PM |
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Always make sure there's a fat man in the lift before you get in, then you can always stand on him in case of emergencies, soften the blow (for
you at least) 
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JoelP
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| posted on 17/1/07 at 09:22 PM |
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sometimes you are best off bracing yourself against the front of a falling/crashing object. Imagine, for example, being in a bus as it run into a
wall. If you are unsupported, you fall forwards with an initial velocity the same as the bus'. You then hit something stationary a few moments
later when the bus is at a complete halt. Splat. However, if you brace yourself against something, you will deaccelerate at the same rate as the rest
of the bus, which will then let you take advantage of any crumple zones the bus may have.
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DIY Si
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| posted on 17/1/07 at 09:24 PM |
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I still prefer the fat man approach! Or grab a large breasted woman! Advisable in a near death situation, as it may cause trouble on the street!
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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907
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| posted on 17/1/07 at 09:58 PM |
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quote:
Originally posted by smart51
If the lift is traveling at 50 MPH when it hits and you have just jupmed up at 10 MPH then you "only" hit the ground at 40 MPH. This is
assuming the lift doesn't bounce at the bottom.
OR...
You hit your head on the roof of the lift at 60mph
Paul G
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Simon
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| posted on 17/1/07 at 10:06 PM |
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quote:
Originally posted by Colnago_Man
.... equal and opposite reaction' (quote I.Newton) this may only increase the speed of the lift, eg you cannot win!
Strictly true, but not quite accurate. You need to bear in mind the mass of the lift vs the lift of the jumping person. If the lift weighs a ton, the
man a tenth of a ton, he will move ten times further than the lift. So the lift increase speed (above typical acceleration, but he will decelerate
slightly less than pushing against a solid floor.
In order for the man to survive the drop, he'll need to be able to accelerate the same amount the lift has, in the time it takes to jump, ie if
lift has been accelerating for 15 secs, it will reach a given velocity (ignoring friction for the minute. The jumper will have to reach that velocity
instantly for the upward jump.
ATB
Simon
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Liam
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| posted on 17/1/07 at 11:10 PM |
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quote:
Originally posted by Donkeymatt
As studied in year 1 Mechanical engineering in Cardiff University!!! If the lift drops at that velocity - you hit the roof at the same velocity, so
the chance of you "standing" on the floor is minimal - you will be pushed very hard against the ceiling, and then catapulted into the
floor at a very high speed when the lift stops! This usually causes a whole load of horrible death in both instances! Who says students don't
learn anything!!!
More study and less beer required!
It's perfectly possible in pure theory. Let's make the lift an order of magnitude or two heavier than the person (a big lift!) so that
when he jumps he receives almost all the change in velocity and the lift a negligable amount. Also, it makes no difference that he's weightless
- he can still push off the lift with all his might (as an astronaut could push off the space shuttle in orbit with all his might - he'll move
away quickly but the shuttle will stay safely in orbit due to it's much larger mass).
All the guy has to do then, is jump upwards at the same velocity as the lift has reached right before the lift hits the ground. If he times it right,
as his toes leave the lift floor, his velocity has cancelled to zero, the lift follows instantly by hitting the floor, and voila - he walks out
unscathed, not even hit the roof of the lift.
Only problem is that if the lift is gonna fall more than a couple of feet, no human is powerful enough to jump fast enough to cancel the falling
lift's velocity. As Phil rightly pointed out, another way of looking at the requirement to cancel the lift's velocity is that you have to
be powerful enough to jump as high as the lift started falling from!!
In reality the most powerful human will take off a tiny amount of the velocity at which he's falling, still be hurtling towards the floor, and
splatter nicely at the bottom.
Liam
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MikeRJ
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| posted on 18/1/07 at 12:05 AM |
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quote:
Originally posted by JoelP
However, if you brace yourself against something, you will deaccelerate at the same rate as the rest of the bus, which will then let you take
advantage of any crumple zones the bus may have.
Absolutely true, but I'm wondering if lifts have crumple zones
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