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**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