Hooke's Law Elevator Spring Question

AI Thread Summary
The discussion revolves around a physics problem involving Hooke's Law and an elevator scenario. A 60-kg person standing on a spring causes its length to change when the elevator accelerates upwards at 2.50 m/s². The calculations show that the spring constant (k) is -7358, and the new length of the spring (L2) is determined to be 0.30m from equilibrium. Confusion arises regarding the coordinate system, particularly whether the spring is displaced upward or downward as the elevator accelerates. Participants emphasize the importance of consistent sign conventions in calculations related to forces and displacements.
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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