Oscillations, velocity, elasticity

[name_ask17]

Homework Statement

A 0.2 kg object, suspended from a spring with a spring constant of k = 10 N/m, is moving in simple harmonic motion and has an amplitude of 0.08 m. What is its velocity at the instant when its displacement is 0.04 m?

Homework Equations

v=root(F/m/l)

The Attempt at a Solution

i did v=root(9.8*.2/.2/.04) and this gave me 15.65, but the answer is 40 cm. how? please help; i do not understand this at all. also, can you please help on thsi question? i feel stupid.
https://www.physicsforums.com/showthread.php?p=3127238#post3127238

 

[RTW69]

For a vibrating spring-mass system the position equation is x=A*cos(w*t) where A=amplitude and w=angular frequency. w = sqrt(k/m). Plug x=.04 into equation to find time when displacement is .04 meters. velocity=dx/dt = -A*w*sin(w*t). Plug time into velocity equation to find instantaneous velocity.