Elevator Free Fall: Understanding the Physics and Sensation of a Cut Cable

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Discussion Overview

The discussion revolves around the physics of an elevator experiencing two scenarios: one where it accelerates downward at 9.8 m/s² and another where the cable is cut. Participants explore the sensations experienced by someone inside the elevator in both situations, examining concepts of free fall, acceleration, and the forces at play.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that in both scenarios, the experience for someone inside the elevator would feel the same due to the acceleration being equivalent to free fall.
  • Others argue that if the elevator accelerates downward at 9.8 m/s², it implies that the cables are not providing support, similar to when the cable is cut.
  • A participant questions whether the 9.8 m/s² acceleration is due to gravity alone or if there are additional forces involved.
  • There is a discussion about the normal force and its relevance in the context of the elevator's motion and forces acting on it.
  • Some participants clarify that while the elevator is in free fall, objects inside will experience weightlessness, contrasting this with an elevator accelerating at 9.8 m/s² in space.
  • One participant suggests that if the elevator is lowered at 9.8 m/s², it should have the same effect as cutting the cable, prompting further clarification on the forces involved.
  • Another participant emphasizes that under free fall conditions, both the elevator and the person inside would accelerate at the same rate, leading to a sensation of weightlessness.

Areas of Agreement / Disagreement

Participants generally agree that both scenarios lead to similar sensations of weightlessness for someone inside the elevator, but there is disagreement regarding the forces involved and the implications of acceleration versus free fall. The discussion remains unresolved on certain technical aspects and interpretations of the forces at play.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about forces acting on the elevator and the definitions of acceleration and free fall. Some participants express confusion about the role of normal force and additional forces in the scenarios presented.

blueblast
Hi guys,

I know this is a really simple question, but I'm just making sure I have this concept down:

Scenario 1. An elevator accelerates down at 9.8m/s^2.
Scenario 2. The elevator cable is cut.

If someone is inside an elevator, scenarios 1 and 2 would feel exactly the same to them, correct?

Thanks,

blueblast
 
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blueblast said:
Scenario 1. An elevator accelerates down at 9.8m/s^2.
Scenario 2. The elevator cable is cut.

If someone is inside an elevator, scenarios 1 and 2 would feel exactly the same to them, correct?
Assuming no other forces on the elevator aside from the cables and the Earth's gravity then you are correct.

If the elevator accelerates downward at 9.8 m/s^2 then it must be getting no support from its cables. Whether this is because the cables go slack or because they have been cut is pretty much irrelevant.
 
blueblast said:
Scenario 1. An elevator accelerates down at 9.8m/s^2.
Scenario 2. The elevator cable is cut.
Is the 9.8m/s2 plus gravity, because if the cable is cut it gets 9.8m/s2 acceleration, anyway... because if you accelerate an elevator at 9.8m/s2 in space would be the same as cutting the cable on Earth.
 
jerromyjon said:
Is the 9.8m/s2 plus gravity, because if the cable is cut it gets 9.8m/s2 acceleration, anyway... because if you accelerate an elevator at 9.8m/s2 in space would be the same as cutting the cable on Earth.
No. Cutting the cable on Earth puts the elevator in free fall: objects inside the elevator will float weightless. Accelerating at 1g in space is the same as the elevator sitting still on the surface of the earth.
 
So,

(normal force) - (force accelerating elevator down) - (gravity) = - (gravity)
?
 
blueblast said:
(normal force) - (force accelerating elevator down) - (gravity) = - (gravity)
?
What normal force?
What force accelerating the elevator down, other than gravity?
And why have -gravity on both sides of the equality?
 
The "force accelerating elevator down" is what is moving the elevator down at an acceleration of 9.8 m/s^2, while the cable is still attached.

I have gravity on both sides since I was comparison Scenario 1 and Scenario 2.

Am I understanding this concept completely wrong?
 
blueblast said:
The "force accelerating elevator down" is what is moving the elevator down at an acceleration of 9.8 m/s^2, while the cable is still attached.
Gravity alone would make the elevator accelerate downward at 9.8 m/s^2. Are you saying that there is a second force in addition to gravity that is pulling the elevator downward at a total of 19.6 m/s^2?
Am I understanding this concept completely wrong?
It is not clear what question you are trying to ask yet.
 
blueblast said:
So,

(normal force) - (force accelerating elevator down) - (gravity) = - (gravity)
?
Show us your free body diagram for this scenario.
 
  • #10
Okay, let me reiterate this whole thing:

So let's say an elevator is hanging by a cable. It is not accelerating. Although there is gravity, the cable supports the weight of the entire elevator(this part I know I'm right for sure). Now let's say this elevator was lowered at an acceleration of 9.8 m/s^2. Since this is the same acceleration as gravity(free fall), would this have the same effect as just cutting the cable of the elevator?
 
  • #11
blueblast said:
So let's say an elevator is hanging by a cable. It is not accelerating. Although there is gravity, the cable supports the weight of the entire elevator(this part I know I'm right for sure). Now let's say this elevator was lowered at an acceleration of 9.8 m/s^2. Since this is the same acceleration as gravity(free fall), would this have the same effect as just cutting the cable of the elevator?
As I said in #2, a slack cable (required to achieve this downward acceleration) and a cut cable have the same effect.

There is other "downward force" involved. And no unidentified "normal force" involved. However, if you want to toss in additional forces on the elevator that all sum to zero, that works too.
 
Last edited:
  • #12
blueblast said:
If someone is inside an elevator, scenarios 1 and 2 would feel exactly the same to them, correct?
Under free fall conditions, the elevator AND the person will be accelerating at the same rate. IF you just 'accelerate the elevator' out in space then the passenger would be left behind and end up against the ceiling of the box - just as if the elevator were suspended (upside down) on Earth. The passenger would not feel weightless.
 
  • #13
Okay, thanks everyone!
 

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