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

In summary, the conversation discusses the concept of an elevator accelerating downward at 9.8m/s^2 and the effects of cutting the elevator cable. It is determined that scenarios 1 and 2, with the elevator accelerating or the cable being cut, would feel the same to someone inside the elevator assuming no other forces are present. The conversation also clarifies the use of the normal force and the forces involved in the elevator's downward acceleration. Ultimately, it is agreed that the passenger inside the elevator would feel weightless in both scenarios.
  • #1
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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  • #2
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.
 
  • #3
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.
 
  • #4
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.
 
  • #5
So,

(normal force) - (force accelerating elevator down) - (gravity) = - (gravity)
?
 
  • #6
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?
 
  • #7
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?
 
  • #8
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.
 
  • #9
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!
 

1. What is elevator free fall?

Elevator free fall is a phenomenon that occurs when an elevator suddenly drops due to a cut cable or malfunction. This results in the elevator and its occupants falling at the same speed as gravity, giving the sensation of weightlessness.

2. How does gravity play a role in elevator free fall?

Gravity is the force that pulls objects towards the Earth. In the case of elevator free fall, gravity is the only force acting on the elevator and its occupants, causing them to accelerate downwards at a rate of 9.8 meters per second squared.

3. What factors determine the speed of an elevator in free fall?

The speed of an elevator in free fall is determined by the acceleration due to gravity and the distance the elevator falls. The longer the distance, the faster the elevator will be moving when it reaches the ground.

4. Is elevator free fall dangerous?

While the idea of being in a falling elevator may seem scary, most modern elevators are equipped with safety mechanisms that prevent free falls from occurring. Additionally, the sensation of weightlessness in free fall is actually less jarring than the abrupt stops and starts of a regular elevator ride.

5. How does understanding the physics of elevator free fall benefit us?

Understanding the physics of elevator free fall can help engineers design safer and more efficient elevators. It also allows for better safety protocols in case of an emergency, and can help alleviate fear and anxiety for those who may experience a free fall in an elevator.

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