Third Law of Motion: Elevator Question

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Discussion Overview

The discussion revolves around the application of Newton's Third Law of Motion in the context of an accelerating elevator. Participants explore the relationship between forces acting on a person in an elevator, the role of the scale, and the implications of acceleration on these forces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that when an elevator accelerates upwards, a person exerts a greater force on the scale, which in turn exerts a greater force back on the person.
  • Others clarify that "weight" pulls down on a person, and the Third Law involves the person pulling on the Earth, indicating that the forces involved do not have to equal the person's weight.
  • There is a question about how a net force exists to accelerate the body if the forces are not equal.
  • Some participants assert that the force of gravity on the body is not equal to the force of the elevator on the body, leading to a non-zero net force.
  • One participant questions the role of the scale and the elevator floor in preventing the scale from accelerating downwards, suggesting that tension in the elevator is a factor.
  • Another participant notes that any downward force on the elevator floor could affect its speed, but inefficiencies such as friction may render these effects undetectable.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of forces in the context of the Third Law and the dynamics of the elevator system. The discussion remains unresolved with multiple competing perspectives on how these forces interact.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the forces involved, the definitions of weight and normal force, and the complexities of the elevator's motion, including friction and tension.

ual8658
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Ok I always understood the 3rd law and normal force well until recently when I started overthinking it. In an elevator, your weight pushes down on say a scale. That scale pushes back onto you with an equal and opposite force. But if the elevator is accelerating upwards, then do you exert an even greater force on the scale and the scale in turn exerts an even greater force back on you? How does the 3rd law apply in an accelerating elevator?
 
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ual8658 said:
But if the elevator is accelerating upwards, then do you exert an even greater force on the scale and the scale in turn exerts an even greater force back on you?
Yes, the same way that you press harder against the seat in an accelerating vehicle.
 
ual8658 said:
In an elevator, your weight pushes down on say a scale.
No, "your weight" pulls down on you. The 3rd Law opposite force to this is you pulling on the Earth.

ual8658 said:
That scale pushes back onto you with an equal and opposite force.
That is another 3rd Law pair. Both are contact forces. None of them is weight. And they don't have to be equal to your weight.
 
So then how is there a net force to force your body to accelerate in that situation?
 
ual8658 said:
So then how is there a net force to force your body to accelerate in that situation?
The force of gravity on your body is not equal to the force of the elevator on your body. The net is non-zero.
 
jbriggs444 said:
The force of gravity on your body is not equal to the force of the elevator on your body. The net is non-zero.
Ok, so there is a net force upward because your weight is not as large as the the force of the scale on you. The force you exert on the scale as part of the 3rd law doesn't really matter right?
 
ual8658 said:
Ok, so there is a net force upward because your weight is not as large as the the force of the scale on you. The force you exert on the scale as part of the 3rd law doesn't really matter right?
Right!

The force you exert on the scale counts toward the net force on the scale, not toward the net force on you.
 
ual8658 said:
So then how is there a net force to force your body to accelerate in that situation?
Do not confuse the Third Law with notions of equilirium and the First Law. The third law is to do with the interaction between two objects and the conservation of Momentum. The First Law is to do with what happens to an object when subjected to one of more forces.
 
I see. Like I said I was overthinking, and the last thing I need to clear up is if you exert a force on that scale, what is stopping that scale from accelerating downwards? The force of the elevator floor? And the elevator is accelerating most likely through tension so that's why the scale doesn't move downwards right?
 
  • #10
ual8658 said:
I see. Like I said I was overthinking, and the last thing I need to clear up is if you exert a force on that scale, what is stopping that scale from accelerating downwards? The force of the elevator floor? And the elevator is accelerating most likely through tension so that's why the scale doesn't move downwards right?
Any downward force on the floor could affect the speed of ascent or descent but the whole machine is very inefficient (lots of friction) and the effect could be undetectable.
 
  • #11
Alright thank you all!
 

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