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# Mechanics pulleys! watch

1. If I want to find the acceleration of the 3m box,
and assuming the direction of the net movement is positive,

I set:

3mg -k(-2h) - mg = 3a

since Tension between the strings cancels out,
there's 3mg acting downwards for 3m box, mg for m box,
and since the Elastic potential energy is F=-kx where k= 3mg/h (given),
and the spring is STRETCHED by 2h thus the negative,

I get 8mg=3a
a= 8mg/3

Am I doing something wrong here?
2. (Original post by liemluji)
If I want to find the acceleration of the 3m box,
and assuming the direction of the net movement is positive,

I set:

3mg -k(-2h) - mg = 3a

since Tension between the strings cancels out,
there's 3mg acting downwards for 3m box, mg for m box,
and since the Elastic potential energy is F=-kx where k= 3mg/h (given),
and the spring is STRETCHED by 2h thus the negative,

I get 8mg=3a
a= 8mg/3

Am I doing something wrong here?
A logical step would be first to attach the actual question for us to see.

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