Finding the equation of a system involving dashpots and mass on wheels

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Discussion Overview

The discussion revolves around finding the equations governing a mechanical system that includes dashpots and a mass on wheels. Participants are attempting to derive the equations of motion based on the provided system diagram and are exploring the relationships between the forces, velocities, and displacements involved.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses uncertainty about how to treat the dashpot in parallel with the mass and presents a system of equations that they believe may describe the system.
  • Another participant references the behavior of dashpots, noting that resistance is proportional to speed, which may influence the equations being derived.
  • A correction is made regarding the initial equations, with a participant acknowledging a typographical error and providing revised equations of motion.
  • A participant suggests using analogies from electrical circuits to understand the mechanical system better, comparing the dashpot to a resistor and the mass to an inductor.
  • Further modifications to the equations are proposed, but the participant expresses continued uncertainty about their correctness.
  • Another participant points out that the input should be considered a velocity input rather than a force input and offers additional insights on the equations involving the mass and dashpot.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the correct formulation of the equations. There are multiple competing views and ongoing corrections, indicating that the discussion remains unresolved.

Contextual Notes

There are indications of missing assumptions regarding the definitions of variables and the nature of the input. The participants have not fully resolved the mathematical steps involved in deriving the equations.

lzh
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Homework Statement


consider the mechanical system below. Find the equation depicting the system. u is the input force. sorry for the poor picture, I had to draw it on my tablet...
http://img685.imageshack.us/img685/1043/ogataprob.png

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Homework Equations


F=ma


The Attempt at a Solution


I'm not sure how to treat the dashpot in parallel with the mass. I came up with the following system of equations:

0=k1x+mx"+k2(x-y)+b(x-y)
0=b(x-y)+k2(x-y)+k3(y-u)
 
Last edited by a moderator:
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hi lzh! :smile:

according to http://en.wikipedia.org/wiki/Dashpot" , the resistance is porportional to the speed :wink:
 
Last edited by a moderator:
hi!
oops I mistyped...
0=k1x+mx"+k2(x-y)+b(x'-y')
0=b(x'-y')+k2(x-y)+k3(y-u)
 
I can't tell you anything for sure since I haven't done these problems in years. The way I used to check out the mechanical "circuit" when I got overwhelmed was based on my electrical knowledge. Your dashpot is a resistor and your mass is an inductor. Always use this as a double check when you're unsure of a mechanical setup. The info for the electrical setup will always be more easy to find.

EDIT: Looking at your equations of motion now though, I think you have it or are close. There is no need to fudge with the circuit. Just find X and Y based on input u.
 
Last edited:
0=k1x+mx"+k2(x-y)+b(x'-y')
0=b(y'-y')+k2(y-x)+k3*y+u

after some more modification, but I'm still unsure...
 
For starters:

Note that u would be a velocity input, not a force input.

b is by' - it has no contact with the velocity at x

and I think for m it is m(y"-x")

I haven't checked your equations yet, so patience please
 

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