Mass-Spring Oscillation question

1. Sep 18, 2006

Mozart

I tried to work out this problem a few different ways but I never get the right answer.

A block hangs in equilibrium from a vertical spring. When a second identical block is added, the original block sags by 8.00 cm

What is the oscillation frequency of the two-block system?

What I've done so far:

Attempt 1:

(Fnet)y=-ky
2mg=-ky
m=((-ky)/(2g))

Then using T=2pi times sqaure root[m/k]

puting in my m as what I found in terms of k, y, and g. The K's cancel and I am left with things I know and then calculate to find my T it turns out to be 0.40 seconds

and then I finish off my problem with freqency= 1/period and get 2.5 Hz

But this is incorrect.

I probably made a wrong assumption but I can't put my finger on it. I hope someone can put me on the right track.

Thank you.

2. Sep 18, 2006

Staff: Mentor

What you want to do is find the spring constant in terms of m, y, & g. Assuming that y is the additional displacement from equilibrium, then:
mg = ky (since one block is added)

What mass goes here? (The system has two blocks now.)

3. Sep 18, 2006

Mozart

Thank you! That worked perfectly. I understand where I made my mistake now too.