# Calculating spring constant

## Homework Statement

A mass 1 kg mass is hung from a spring.
Calculate the spring constant from the given data. Velocity initial= 3.0 m/s Velocity final= 1.5 m/s. The initial height traveled is 0.1 meters and the final is 0.25 meters. These height values will be negative since i picked my starting point at the origin so they have moved below the X making them negative.

## Homework Equations

I know i am supposed to use the conservation of energy equation with the potential elastic factored into the total PE. the equation would be (1/2*m*v^2)+(mgh)+1/2*k*x^2)=(1/2*m*v^2)+(mgh)+1/2*k*x^2) where the left is the initial and the right is the final. However i keep getting the wrong answer. I think I am doing a simple algebraic error in my calculations. any guidance is appreciated

## Answers and Replies

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jack action
Gold Member
I think you have to determine the initial displacement of the spring under the static load:

Xo = mg / K

Then final displacement is:

Xf = mg / K - $$\Delta$$X

Where $$\Delta$$X = 0.25m - 0.1m = 0.15m

Put that in your equation and do the transformations and it should give you the correct answer.