Is jumping up in a falling elevator possible?

  • #1
Imagine that you are using an elevator and - due to some cause - all of the cables supporting it cracked and the brakes were not engaged - the elevator is in a free fall. Now, would it be technically possible to jump in that elevator?

Jumping requires the force of reaction of the ground, this implies that we need to exert a force on the floor in the elevator.

Assume no or little air resistance - we are in a free-fall, therefore the normal force acting on our body is $0$ therefore, it is not possible to jump.

However, if the air resistance were larger, we would not experience air resistance and so we would exert a force on the floor of the elevator, receiving the normal force due to the third rule of motion. In this case, jumping would be possible.

Therefore, does it mean that in real life situations, it is possible to jump in a falling elevator?
 

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  • #2
jbriggs444
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Therefore, does it mean that in real life situations, it is possible to jump in a falling elevator?
No air resistance is required. If you can crouch first (e.g. by holding onto the rails and pushing yourself down) then you can jump. If you cannot crouch, you may be able to push off with your toes. If you are floating freely in mid-elevator out of reach of floors, walls or ceiling, you will have difficulty, however.

Astronauts in a space capsule or space station can jump off the floors or walls just fine.
 
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  • #3
Ibix
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As jbriggs444 says, this is essentially the same situation as astronauts in free fall. You may wish to look up the "vomit comet", a 747 that can go ballistic for a while to allow free fall training and filming of things like the movie Apollo 13.

Air resistance inside the lift won't really do much, although I gather that it's possible to swim a bit in air. However, the lift itself will experience air resistance as it picks up speed. A lift shaft is nowhere near long enough for this, but if it were to fall for long enough air resistance would eventually match the force of gravity. This is the so-called "terminal velocity". This would manifest itself inside the lift as weight returning, eventually to 1g which would remain constant (neglecting turbulence, tumbling etc) until impact.
 
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  • #4
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As jbriggs444 says, this is essentially the same situation as astronauts in free fall. You may wish to look up the "vomit comet", a 747 that can go ballistic for a while to allow free fall training and filming of things like the movie Apollo 13.
A nitpick... this aircraft wasn't a Boeing 747, it was a KC-135 based on the much older narrow-body Boeing 707 airframe.
 
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  • #5
rcgldr
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Skip to 0:15 in this old Skylab video:

 
  • #6
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Imagine that you are using an elevator and - due to some cause - all of the cables supporting it cracked and the brakes were not engaged - the elevator is in a free fall. Now, would it be technically possible to jump in that elevator?

Jumping requires the force of reaction of the ground, this implies that we need to exert a force on the floor in the elevator.

Assume no or little air resistance - we are in a free-fall, therefore the normal force acting on our body is $0$ therefore, it is not possible to jump.

However, if the air resistance were larger, we would not experience air resistance and so we would exert a force on the floor of the elevator, receiving the normal force due to the third rule of motion. In this case, jumping would be possible.

Therefore, does it mean that in real life situations, it is possible to jump in a falling elevator?
In an elevator that is in an area of gravity(the Earth), the floor is effectively the ground, gravity pulls you to the Earth, pulling you to the floor of the elevator. If you are contacting the floor when the cables break, you and the elevator will both accelerate toward the center of gravity at the same rate, you will not lose contact with the floor unless a force is applied in some vector away from center of gravity.......such as jumping......, forces applied in some vector towards the center of gravity(such as squatting in order to jump) are not impeded because they do not resist gravity, they work with gravity, squatting only helps gravity pull you closer to the floor, which is what it wants to do anyway. As stated by another here, if you can squat then it stands to reason that you could jump unless there is some force stopping you from straightening your legs.


Note, jumping in this scenario moves you away relative to the floor of the elevator, but may not move you away relative to the earth unless you are jumping away from center if gravity with more velocity than the velocity at which gravity is pulling you.


Edit: disregard, lol.
 
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  • #7
Ibix
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forces applied in some vector towards the center of gravity(such as squatting in order to jump) are not impeded because they do not resist gravity, they work with gravity, squatting only helps gravity pull you closer to the floor, which is what it wants to do anyway.
The problem is that in freefall, squatting will simply lift your feet off the floor and leave you floating in the air unless you are stuck down somehow (glue, magnets, caught your foot under something bolted down).
 
  • #8
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The problem is that in freefall, squatting will simply lift your feet off the floor and leave you floating in the air unless you are stuck down somehow (glue, magnets, caught your foot under something bolted down).
I get it. Something would have to accelerate your whole body down at a faster velocity relative to your feet(or the floor/center of gravity) in order to squat, attempting to squat only lifts your feet toward you, our feet would have to have more gravity than the elevator in order to pull the elevator toward the body along with our feet, plus enough gravity to pull the elevator toward us(away from center of gravity) with more force than the center of gravity is pulling on the elevator. If our feet had more mass than the rest of our body, would that still be true? Its easier for the feet to come toward the body than it is for the body to come toward the feet(least resistance). Or does center of mass even come into play?

@Ibix
 
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  • #9
jbriggs444
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If our feet had more mass than the rest of our body, would that still be true? Its easier for the feet to come toward the body than it is for the body to come toward the feet(least resistance). Or does center of mass even come into play?
The mass of the feet does not matter. The center of mass stays where it is relative to the freely falling frame.
 
  • #10
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If all security systems fail, and if the elevator height is enough when it begins to fall, then it could reach the terminal velocity, due to the air resistance and perhaps some friction from the guide rails, before crashing.

In a Mythbuster Discovery episode, they dropped an elevator cage from 4 or 5 floor height, and recorded a 85 Km/h final speed (which is 23.6 m/s).

So, imagine that very cleverly, you at the last moment jump, and you being in a good fit, are able to jump 70cm height in normal conditions. If you do the numbers, that height corresponds to a speed of about 3.5 m/s.

Then, you finally "crash" speed would be 20 m/s, instead of 23.6 m/s ... In you grave stone you could write: "At least, I tried".
 
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  • #11
At zero friction outside and you originally standing in the elevator before it falls, my guess would be that you can't jump intuitevely. But you could probably use the sidewalls of the elevator to push yourself to the ground. And then jump up again.
 
  • #12
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If all security systems fail, and if the elevator height is enough when it begins to fall, then it could reach the terminal velocity, due to the air resistance and perhaps some friction from the guide rails, before crashing.

In a Mythbuster Discovery episode, they dropped an elevator cage from 4 or 5 floor height, and recorded a 85 Km/h final speed (which is 23.6 m/s).

So, imagine that very cleverly, you at the last moment jump, and you being in a good fit, are able to jump 70cm height in normal conditions. If you do the numbers, that height corresponds to a speed of about 3.5 m/s.

Then, you finally "crash" speed would be 20 m/s, instead of 23.6 m/s ... In you grave stone you could write: "At least, I tried".
This is a Youtube link to the Mythbusters video.
 
  • #13
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Lying flat sounds like a bad idea, since the collision will be quicker and the forces stronger. If you stand up and do not lock your knees, at least some expendable parts of you have distance and time to crumple up, absorbing some of the energy, before the vital organs hit hard surfaces. This is standard advice for parachutists who must absorb the remaining energy of falling when they hit the ground -- even with a parachute you do not hit the ground with zero speed, and it is good to absorb it by bending legs and rolling after the impact.
 
  • #14
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This is the so-called "terminal velocity". This would manifest itself inside the lift as weight returning, eventually to 1g which would remain constant (neglecting turbulence, tumbling etc) until impact.

This seems, to me at least, entirely counter-intuitive so I'll need to think it through. It might even motivate my first parachute jump, I'm very much a learn-by-doing type!
 
  • #15
Ibix
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This seems, to me at least, entirely counter-intuitive so I'll need to think it through. It might even motivate my first parachute jump, I'm very much a learn-by-doing type!
When you stand on the floor, gravity pulls you down and the floor pushes you up and you don't accelerate. In a lift or a parachute at terminal velocity, gravity pulls you down and air resistance pushes you up and you don't accelerate. It must feel the same as being on the Earth's surface (or dangling from a harness attached to a crane in the case of the parachute).

Good luck with the parachute jump. My brother in law did a tandem jump once and had great fun. The organisers were really impressed with him because they didn't have to prize his fingers off the plane's exit hatch or anything. Apparently that's quite rare in first timers.
 
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  • #16
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The organisers were really impressed with him because they didn't have to prize his fingers off the plane's exit hatch or anything. Apparently that's quite rare in first timers.

Now that's motivation :biggrin:

As for the 1g at terminal velocity, it's still not clicking. If your speed is not increasing, there is no net acceleration acting, I get that. But the downward force of gravity and the upward force of the elevator floor...that I still need to get my head around, because my monkey brain is saying there would be no weight at all. It'll come - eventually.
 
  • #18
vanhees71
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I don't see, why you should not be able to push yourself up in a free-falling elevator. Also the astronautes in the international space station can move by "pushing" a wall. They just move in uniform motion as predicted by Newton's 1st law (to a good approximation).
 
  • #19
Ibix
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But the downward force of gravity and the upward force of the elevator floor...that I still need to get my head around, because my monkey brain is saying there would be no weight at all. It'll come - eventually.
There's nothing special about terminal velocity. It's just like being in any lift at constant velocity - you only feel heavier or lighter during acceleration phases, not during the constant speed phase. It's just that the force on the lift making it move at constant speed comes from air resistance instead of the lift cable.
 
  • #20
Ibix
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I don't see, why you should not be able to push yourself up in a free-falling elevator.
Bending your knees in preparation for the jump will lift your feet off the floor in free fall, so you'd need something to hang on to initially to set up the jump (unless you were already crouching before the fall started). But after that, agreed. (Edit: as noted upthread, as you probably saw)
 
  • #21
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Lying flat sounds like a bad idea, since the collision will be quicker and the forces stronger. If you stand up and do not lock your knees, at least some expendable parts of you have distance Sarkari Result Pnr Status 192.168.1.1 and time to crumple up, absorbing some of the energy, before the vital organs hit hard surfaces. This is standard advice for parachutists who must absorb the remaining energy of falling when they hit the ground -- even with a parachute you do not hit the ground with zero speed, and it is good to absorb it by bending legs and rolling after the impact.
even with a parachute you do not hit the ground with zero speed, and it is good to absorb it by bending legs and rolling after the impact. like this
 
  • #22
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I don't see, why you should not be able to push yourself up in a free-falling elevator. Also the astronautes in the international space station can move by "pushing" a wall. They just move in uniform motion as predicted by Newton's 1st law (to a good approximation).
In the ISS the astronauts move around slowly, relative to the ISS. If the ISS came to a sudden stop, as in an elevator hitting the bottom of the shaft, the astronauts would still be travelling at about 17,000 mile per hour, relative to the ISS, when they hit the walls.
A similar scenario applies to the elevator and, although the speeds are not as great, the impact speed is very close to the terminal speed of the elevator.
 
  • #23
sophiecentaur
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As jbriggs444 says, this is essentially the same situation as astronauts in free fall. You may wish to look up the "vomit comet", a 747 that can go ballistic for a while to allow free fall training and filming of things like the movie Apollo 13.
A nitpick... this aircraft wasn't a Boeing 747, it was a KC-135 based on the much older narrow-body Boeing 707 airframe.
I think a tad more nit-picking is appropriate here. I have no written evidence about this but I would be surprised if the Americans had used the name of a British Airliner (the Comet - possibly the first civil jet airliner) to describe the procedure if the zero g demonstration to passengers had first been given in an American plane. The world's aeroplane industry is known for its chauvinism and an equally graphic name would have been invented to suit an american plane if it had been the first. (Can't say that I can think of one, offhand, though)
 
  • #24
DaveC426913
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Note BTW, that in a - practical situation - the elevator's terminal velocity will surely be demonstrably less than your fall within the car - due to the column of air trapped in the elevator shaft.

It should certainly be enough to have your feet back on the floor in time to push off.
 
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  • #25
DaveC426913
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I think a tad more nit-picking is appropriate here. I have no written evidence about this but I would be surprised if the Americans had used the name of a British Airliner (the Comet - possibly the first civil jet airliner) to describe the procedure if the zero g demonstration to passengers had first been given in an American plane.
I don't follow. Are you saying they would have avoided the word "Comet" because a British plane has that name? Well they didn't avoid it.
Not sure I understand your point.
 

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