# Friend pulls the spring without you looking

## Homework Statement

0.500kg mass resting on frictionless surface is attached to a horizontal spring with a spring constant of 45N/m. When you are not looking, your lab partner pulls the mass to one side and then releases it. When it passes the equlibrium position, its speed is 3.375m/s. How far from the equilibrium position did your lab partner pull the mass before releasing it?

## Homework Equations

Elastic Potential Energy
Kinetic Energy

## The Attempt at a Solution

What is confusing me is how I am going to apply the law of conservation of energy here. Could I say that whilst he is pulling the mass, only Ee is present and not Ek?

I was thinking I would do something like this to end up getting the "x" value needed:

1/2(k)(x)^2 = 1/2(m)(v)^2 <-- then solve for x, which is the amount of stretch or compression..

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What is confusing me is how I am going to apply the law of conservation of energy here. Could I say that whilst he is pulling the mass, only Ee is present and not Ek?
Assume that when he releases the mass it starts from rest.

I was thinking I would do something like this to end up getting the "x" value needed:

1/2(k)(x)^2 = 1/2(m)(v)^2 <-- then solve for x, which is the amount of stretch or compression..
Sounds good to me.