Understanding Forces in Coupled Oscillating Systems

[rsaad]

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.

 

[tiny-tim]

welcome to pf!

hi rsaad! welcome to pf!

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

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?)

 

[rsaad]

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.

 

[tiny-tim]

sorry, i can't follow this

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

how long are OA AB and BC ?​

 

[rsaad]

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

 

[tiny-tim]

but A and B are moving

 

[rsaad]

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