Simple harmonic motion with an amplitude

[physics_geek]

Homework Statement

A block of unknown mass is attached to a spring with a spring constant of 5.50 N/m and undergoes simple harmonic motion with an amplitude of 12.5 cm. When the block is halfway between its equilibrium position and the end point, its speed is measured to be 25.0 cm/s.

(a) Calculate the mass of the block.
(b) Calculate the period of the motion.
(c) Calculate the maximum acceleration of the block.

Homework Equations

Vmax = sqrt(k/m)A
T= 2pi/w
a_max = kA/m

The Attempt at a Solution

so i thought i would use the first formula i stated to find the mass of the block
but apparently I am wrong.
can someone please help me get started.
thanks

 

[rock.freak667]

Well the total energy of the system is given by 1/2 kA² and throughout the motion this is interchanged between elastic potential and kinetic energy. So halfway between the max point and equilibrium, how far does the spring stretch? Can you now apply a conservation of energy to get the mass?

 

[rl.bhat]

Show your calculations to see where you have made mistake.

 

[physics_geek]

well what i did was
vmax = sqrt(k/m)A
(v^2/A^2)m = k
so
m = kA^2/v^2 but i guess this is wrong

so do i use something like 1/2kA^2 = 1/2mv^2 ?