# Simple harmonic motion ?

#### physics noob

simple harmonic motion ???

hey guys, im having troube with this problem

a vertical spring stretches .25m when a 2kg weight is hung from it. If that weight were oscillating up and down, how long would it take to complete 10 cycles?

so first i tried to find k by using PEi + KEi + PE(spring)i =PEf etc

so 2kg(.25m)(9.8) = .5(k)(.25)^2

so k= 156.8

then finding T T= 2(pi) sqroot(m/k)

so i get T= .709 * 10 oscillations = 7.1sec

correct answer is 10sec, meaning T must equal 1sec, which means k should equal somewhere around 78 or 79 .....working backwards....
i dont see how they get this value...............any help is greatly appriciated THANKS

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#### dextercioby

Homework Helper
Hmm. Why can't you get the "k" value from the second law of dynamics applied to the spring-mass system...?

Daniel.

#### physics noob

hahaha, right on man, thanks!!! also thanks for not making me look like an idiot, i totally spaced using F=ma,,,,,however my question still remains, is it possible using MEi = MEf to obtain spring constant? thanks again for the quick help....

#### Doc Al

Mentor
physics noob said:
a vertical spring stretches .25m when a 2kg weight is hung from it.
From that fact and Hooke's law you should be able to calculate the spring constant.

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