How to Calculate the Spring Constant for Simple Harmonic Motion?

[physics noob]

simple harmonic motion ?

hey guys, I am 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 don't see how they get this value...any help is greatly appriciated THANKS

 

[dextercioby]

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]

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.