Accelerating Lift Homework: Equations of Motion

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Homework Statement

A light spring force const. k hands from ceiling of an accelerating lift with a block of mass m attached to its bottom end. Block is held stationary (in lift frame) with the string at its equilibrium length while the lift is accelerating at a rate Ag where A is between -1 and 1 (const.) At t=0 the block is released from rest allowing it to execute SHM. Denote X by distance of block from origin of intertial FoR and Xo its distance when the spring is at equil. length..

write down eqn of motion of block in interial FoR..

then more qs

Homework Equations

The Attempt at a Solution

Im quite confused.
So taken intertial FoR to be the ground. but if block is stationary when spring is at equilibrium length and lift accelerating at Ag then surely that means A = -1?

help? Thanks

 

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Thoughts? This is quite urgent sorry!

 

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Is it k(Xo-X) - mg = mx''?

 

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Actually I get (Xo-X)'' + k/m(Xo-X) = g

I know Xo'' = Ag

but how do i solve this eqn of motion?