Hooke's Law Elevator Spring Question

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Homework Help Overview

The discussion revolves around a physics problem involving Hooke's Law and the behavior of a spring in an elevator scenario. The original poster describes a situation where a 60-kg person stands on a spring in an elevator that accelerates upwards, leading to questions about the spring's length and the implications of the coordinate system used.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • The original poster attempts to analyze the forces acting on the spring and the person, using equations related to net force and spring force. They express confusion regarding the signs used in their calculations and how these relate to their defined coordinate system.

Discussion Status

Participants are actively engaging with the original poster's reasoning, questioning the consistency of the signs in their calculations. Some suggest revisiting the coordinate system to clarify the direction of forces and displacements. The discussion is ongoing, with no clear consensus reached yet.

Contextual Notes

The original poster mentions that their professor provided an answer but did not show the work, which adds to their confusion regarding the problem setup and the interpretation of results. There is an emphasis on understanding the implications of acceleration in the context of the spring's behavior.

MMVS
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Homework Statement


Elevator initially at rest.
Equilibrium length L0=40.0cm
60-kg person stands on spring
L1= 32.0cm

The elevator than speeds upwards at 2.50 m/s2
What is the new length (L2)

Homework Equations


Fnet=ma
Fsp=-kdeltaX
FG=mg

The Attempt at a Solution


Taking down as positive y hat direction
Before:
Fnet=Fsp+FG
0=-k(0.32-0.40)+(60.0)(9.81)
K=-7358
After:
Fnet=Fsp+FG
ma=-kdeltaX+mg
m(-2.5)=-(-7358)(L2-0.40)+60(9.81)
L2=[{60(-2.5)-60(9.81)}/-(-7358)]+0.40
=0.30m from equilibrium

I am confused since i don't know how this makes sense according to the coordinate system. (I know the answer is right as my prof posted the answer, not the work, just answer)
At rest change in x = -0.08
When accelerating change in x= -0.10
But since my coordinate system is set up so Y-hat is positive is the downward direction doesn't this mean my spring is being displaces upward?
This doesn't make sense to me , can someone please explain very clearly?
 
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MMVS said:
Fnet=Fsp+FG
0=-k(0.32-0.40)+(60.0)(9.81)
K=-7358

Take a look at your signs again. Are they consistent with the defined direction?
 
JeremyG said:
Take a look at your signs again. Are they consistent with the defined direction?
I kinda see what you are getting at
 
MMVS said:

Homework Statement


Elevator initially at rest.
Equilibrium length L0=40.0cm
60-kg person stands on spring
L1= 32.0cm

The elevator than speeds upwards at 2.50 m/s2
What is the new length (L2)

Homework Equations


Fnet=ma
Fsp=-kdeltaX
FG=mg

The Attempt at a Solution


Taking down as positive y hat direction
Before:
Fnet=Fsp+FG
0=-k(0.32-0.40)+(60.0)(9.81)
K=-7358
After:
Fnet=Fsp+FG
ma=-kdeltaX+mg
m(-2.5)=-(-7358)(L2-0.40)+60(9.81)
L2=[{60(-2.5)-60(9.81)}/-(-7358)]+0.40
=0.30m from equilibrium

I am confused since i don't know how this makes sense according to the coordinate system. (I know the answer is right as my prof posted the answer, not the work, just answer)
At rest change in x = -0.08
When accelerating change in x= -0.10
But since my coordinate system is set up so Y-hat is positive is the downward direction doesn't this mean my spring is being displaces upward?
This doesn't make sense to me , can someone please explain very clearly?
If you're standing in an elevator when it starts to go up, do you feel heavier or lighter when the car starts to move? Is the spring therefore going to get longer or shorter as a result?

You seem to have sprinkled a generous helping of negative signs throughout your calculations without systematically considering coordinate systems.
 

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