# Forces in coupled system

1. Oct 5, 2012

Hi

How do you determine the forces in a coupled oscillating systems? (check the attachment for coupled oscillating system)

I do get the forces right but I make mistakes with the signs.

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2. Oct 5, 2012

### tiny-tim

welcome to pf!

i can only suggest that you draw a very clear diagram, carefully labelling the distances, and then write out a constraint equation such as a + b + c = L

(do you want to show us an example of your going wrong?)

3. Oct 5, 2012

Thank you tinu-tim! =)
Consider the 2nd diagram in the attachment and assume that it is attached on both ends and that x1 is extension in a and x2 is extension b.
for mass a:

-k (x1)
k(x1-x2) <--- now i am not sure whether to put a negative sign with it or positive

for mass b:
-k (x2)
k(x2-x1) <--- again i do not know what sign to put here.

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4. Oct 5, 2012

### tiny-tim

call the points O A B C, with OC = L

how long are OA AB and BC ?​

5. Oct 5, 2012

We haven't been specific about the length in class but lets assume that L/3 = oa= ab= bc

6. Oct 5, 2012

### tiny-tim

but A and B are moving

7. Oct 5, 2012

It is a screen shot at a time when the springs are stretched. I just need to understand which direction the forces apply.