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Simple harmonic motion ?

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


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

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

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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