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Okay this problem is giving me a hard time.. i don't know why i must be missing something...

Here is what I have done, i said that at the beginning KE is 0 and PE must be 90 J.

Once it falls down the 7.5m the KE is 90J therefore the PE is 0J.

There is the "3 seconds time" that is there and i know im supposed to use it somehow but i could use some help here... i calculated the acceleration with that but fail to see whre i could use it right now...

here are my equations... for the beginning:

initial

total energy =90 = PEi + KEi = 7.5(9.8)(m1+m2) + 0

final

total eneryg = 90 = PEf + KEf = ((m1+m2)(v^2))/2

this homework problem is due in 3 hours, its the last problem i have to do , just a hint would help a lot, thanks guys