# Homework Help: Spring - Damper - Mass System (in series)

1. Jun 30, 2011

### LittleFill

Spring - Damper - Mass System in series

fixed-----Spring-----Damper-----Mass----->F
c d m
--> -->
x1 x2

I need help setting up the equations, I know it has to have 2 subsystems but I just can't figure out the two equations I'm supposed to get.

2. Jul 1, 2011

### xHossx

Could it be something like:

m d(x1 + x2)/dt^2 = F - d dx2/dt

c x1 = d dx2/dt

As in:

Mass x acceleration = external force - counteracting damper force
Spring force = damper force (= reaction force on the fixed 'wall')

With the origin of the X axis at the fixed wall (duh) and its plus direction pointing to the right.

Took x1 and x2 to be the lengths of the spring and damper, respectively. ('Picture' isn't too clear.)
Just in case they're really the positions of the spring-damper midpoint and the mass, well... replace x1 + x2 with x2 and wherever it says x2 above should become x2 -x1...

3. Jul 1, 2011

### LittleFill

yea it is after stayin a while in the library and looking at a couple of books i finally figured it out.
and it was x1-x2 and x2-x1 i really should have looked at my drawing again after it was posted it was much clearer before i pressed on submit (alot of empty spaces weren't there anymore)

thank you!