What is the maximum speed of a 4.00kg mass attached to a spring with k=20.0N/m?

[Gokuraku]

Homework Statement

I'm sorry to bother anyone but this question is really driving me insane. It's one of my question on my notes and I've been trying to re-solve it.
It's in oscillatory unit and apparently we have to know what the speed is.

A 4.00kg mass is an horizontal frictionless surface attached to a spring with k=20.0N/m. The mass is pulled 0.300m from a equilibrium, then let go.

Homework Equations

max speed

The Attempt at a Solution

The answer is suppose to be 0.671m/s but I keep getting 0.612m/s...

..Never mind I figured it out.

 

[raindrops]

Wspring=(1/2)kx^2
so W=.5*20*.3^2=.9

since the max speed is when all the energy is kinetic you can say the Wspring=(1/2)mv^2
so .9=.5*4*v^2

.45=v^2
sqrt(.45)=.6708 or .671

 

[Gokuraku]

Wspring=(1/2)kx^2
so W=.5*20*.3^2=.9

since the max speed is when all the energy is kinetic you can say the Wspring=(1/2)mv^2
so .9=.5*4*v^2

.45=v^2
sqrt(.45)=.6708 or .671

Thank you for replying, I'm very happy but I already solved it.

I just remember, when you told me, that kinetic energy is in there since it's in motion. Again, thank you and I'm very sorry.