Solve Mass of Glider with Harmonic Motion Help

[elsternj]

Homework Statement

On a frictionless, horizontal air track, a glider oscillates at the end of an ideal spring of force constant 2.20 N/cm . The graph in the figure shows the acceleration of the glider as a function of time.
Find the mass of the glider.

Homework Equations

T= 2pi$$\sqrt{m/C}$$

The Attempt at a Solution

2.2 N/cm = 220 N/m
C=220

.4 = 2pi$$\sqrt{m/220}$$
m = 1389

I know I'm doing something wrong but what?

 

[Quinzio]

How much is the period of the sine wave ?

 

[elsternj]

would it be .2s? when it passes back through O and to where it started?

 

[elsternj]

I tried .2 and .3. are any of these the right time for period? If so then my problem lies elsewhere. any insight? thanks

 

[elsternj]

okay i see now. .10 is the time for a period.

so .10 = 2pi $$\sqrt{m/220}$$

multiply both sides by $$\sqrt{220}$$

1.48 = 2pi$$\sqrt{m}$$

divide both sides by 2pi

2.43 = $$\sqrt{m}$$

square both sides

m = 5.43 which is still the wrong answer.

is my math wrong? what exactly am i doing wrong?