Hi,may potential, time dependent force field be called

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Discussion Overview

The discussion revolves around the nature of time-dependent force fields and whether they can be classified as conservative. Participants explore implications for mechanical energy conservation in isolated systems under such conditions.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant questions if a time-dependent potential force field can be considered conservative and suggests that mechanical energy conservation may not hold in such a field.
  • Another participant explains that a time-dependent force field can be viewed as a function of four variables, complicating the definition of curl, which is traditionally defined in three dimensions.
  • A different viewpoint is presented that while a force may be conservative at a fixed moment in time, the overall energy conservation does not apply to time-dependent forces.
  • One participant seeks clarification on whether the original poster (OP) is suggesting that the force field is conservative at each instant, leading to a discussion about the implications of a time-dependent potential function on the conservation of energy.
  • It is argued that a time-dependent potential function cannot yield a conservative force, as the explicit time dependence in the Lagrangian leads to a non-conserved energy function.

Areas of Agreement / Disagreement

Participants express differing views on the classification of time-dependent force fields as conservative, with no consensus reached regarding the implications for energy conservation.

Contextual Notes

Participants highlight the complexity introduced by time dependence in force fields and the limitations of traditional definitions of conservative forces, but do not resolve the mathematical implications or assumptions involved.

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Hi,

may potential, time dependent force field be called conservative?

If so, the mechanical energy conservation of an isolated mechanical system does not hold in such field?

Thanks.
 
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time dependent means your force field is a function of 4 variables. In math, its equivalent to having a vector field in 4-space. Curl is defined only on R3, but there are some generalizations to the product involving exterior derivatives (which I have not studied).
But when you do a path integral, you have a curve in 4 space which doesn't make physical sense since you can only traverse time in 1 direction, so a closed path is impossible.
 


Curl,
I don't think that we need to incorporate time in definition of conservative force (its just not there). The force may be conservative in a fixed time moment. I don't have problem in calling time dependent force conservative, however the energy conservation will not work on for such force imho.
 


Ok now I think I see what you're saying. Are you considering a vector field which changes with time, but at any instant in time the force field is conservative, i.e. curl F = 0 at a fixed t?
 


Curl said:
Ok now I think I see what you're saying. Are you considering a vector field which changes with time, but at any instant in time the force field is conservative, i.e. curl F = 0 at a fixed t?

I think that's what the OP means.

If that is indeed the case:

A time dependent potential function will not give rise to a conservative force. If we consider the Lagrangian of a system:

[tex]L=T-V(t)[/tex]

Since, V has an explicit time dependence, L has an explicit time dependence. We can also derive the time dependence of the energy function from Lagrangian mechanics:

[tex]\frac{dH}{dt}=-\frac{\partial L}{\partial t}[/tex]

Thus, if L has an explicit time dependence, the energy function will vary in time, and is thus not conserved.
 

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