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## Homework Statement

The scale of a spring balance that reads from 0 to 20.7 kg is 14.9 cm long. A package suspended from the balance is found to oscillate vertically with a frequency of 1.51 Hz. (a) What is the spring constant? (b) How much does the package weigh?

## Homework Equations

F=-kx

## The Attempt at a Solution

I know the answer, I'm not sure why it is done that way. You set kx=mg and solve for k, where x is the length of the spring. Why is that the x value? I thought the spring force only equaled gravity at the equilibrium point? Certainly the equilibrium point will change from the original length of the spring when you hang a mass from it.

Also, why is F=-kx some places, and others you just use F=kx, such as this problem?