# Homework Help: Dot product of Force and Position as a constant of motion - physical significance?

1. Mar 8, 2012

### sam guns

Reason I posted this in the maths help forum is that an equation of this form randomly popped up in a homework I was doing on differential geometry. I started with a one-form ω=dβ (β is a scalar function) and found that if for a random vector v, ω(v) = 0, then

$\frac{d}{dt} \left( \gamma^{i}\frac{\partial\beta}{\partial x^{i}} \right) = 0$

where γ is the integral curve of v (aka the position if you interpret v as a velocity)

If you interpret the scalar field β as a potential field, then this says that the dot product of position and force is a constant of motion. Understanding it is not really significant to what I am expected to turn in, but regardless, does it have any physical significance?
1. The problem statement, all variables and given/known data

2. Mar 8, 2012

### tiny-tim

welcome to pf!

hi sam! welcome to pf!

it looks like the formula for a bead sliding along a frictionless rod forced to rotate (irregularly) about a pivot

but, so far as i know, it has no practical significance​

3. Mar 8, 2012

### sam guns

Re: Dot product of Force and Position as a constant of motion - physical significance

Thanks for your reply! It's kind of what I suspected, for a second I thought it could be some important constant of motion related to the virial theorem or something like that, but I couldn't find anything in my old mechanics textbooks. I guess it's just a curiosity then :)

Last edited: Mar 8, 2012
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