Method to calculate work while moving two walls

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Discussion Overview

The discussion revolves around the calculation of work done while moving two walls (x and y) of a triangular configuration, with a focus on the implications of constant area and pressure conditions. Participants explore the theoretical framework and assumptions behind the mechanics of the system, including the role of an external device and the behavior of gas within the triangle.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes a method for calculating work based on the geometry of a triangle and the behavior of walls under constant area and pressure conditions.
  • Another participant questions the validity of the assumptions, particularly regarding the constancy of volume and pressure, and challenges the notion of work being done in the absence of volume change.
  • Some participants argue that the external device must provide energy to move wall z, while others contend that no work is done if there is no change in volume or pressure.
  • There are discussions about the implications of the geometry on the ability to maintain constant volume and whether the problem is worth pursuing further.
  • One participant suggests that for small displacements, the work can be calculated based on the swept volume of the walls, leading to a discussion about energy input and output in the system.
  • Another participant emphasizes that the work done is external to the walls and depends on the forces applied by the external device.
  • Concerns are raised about the feasibility of maintaining constant volume with the given geometry, with some participants suggesting that it may only be achievable over a limited range.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the calculation of work and the assumptions about pressure and volume. There is no consensus on whether the proposed method is valid or if the conditions allow for meaningful calculations.

Contextual Notes

Limitations include the unclear feasibility of maintaining constant volume with the described geometry, the dependence on the external device's control, and the unresolved implications of pressure differences on the calculation of work.

Who May Find This Useful

This discussion may be of interest to those studying mechanics, thermodynamics, or engineering principles related to work and energy in systems involving gas and pressure dynamics.

  • #31
CWatters said:
Why would they consume any energy?

Work = force * displacement

When everything is stationary there is no displacement going on so no work is being done.

When things are moving the net force is zero (see above) so again no work is done.

I'm assuming there is no friction and the motors are "ideal".

Nothing move ? Look at that image:

tr6.png


At start, the position is 1, at final it is 2. The motor 1 take the black wall. The motor 2 take the pink wall. The motor 3 take the green wall. The black arm turns, no ? the green wall moves, no ? for me the motors 1 & 2 recover an energy. The motor 3 needs the energy recovered by the motors 1 & 2.
 
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  • #32
@Mary:
This argument seems to be along the same lines as the arguments people try to use to prove that they have invented a Perpetual Motion Machine. You have made a highly complicated experiment and done some maths which seems to produce a strange result.
1. No net work is done
2. Some net work is done
?
It's all lost in the fog of your over complicated model. Do you think that, somehow, a complicated enough model will produce a result that proves the gas laws are wrong? Personally, I would always look for a (possibly subtle) error in the model if it yields a result that goes against conventional Science.
 
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  • #33
Mary2100 said:
Nothing move ? Look at that image:

tr6.png


At start, the position is 1, at final it is 2. The motor 1 take the black wall. The motor 2 take the pink wall. The motor 3 take the green wall. The black arm turns, no ? the green wall moves, no ? for me the motors 1 & 2 recover an energy. The motor 3 needs the energy recovered by the motors 1 & 2.

Yawn. Go back and read my post again. I described two situations... one while it's stationary and one while it's moving.

I think I'm just about done with this thread. All the information you need to understand the problem has been well covered.

I'll just repeat.. In in order to work out the energy required to move any of the walls you need to consider all the forces acting on the wall. You haven't done that.
 
  • #34
This thread will be closed unless the next post by the OP is extremely lucid and helps us to clear up your confusion. Fair warning.
 

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