What Are the Units for Precession Rate (Ω)?

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Homework Help Overview

The discussion revolves around the units for the precession rate, denoted as Ω, in the context of gyroscopic precession. Participants are examining the equation Ω = Mgr/Iω and its implications for unit analysis, particularly focusing on how radians are treated in these calculations.

Discussion Character

  • Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants are calculating the units for Ω and questioning the treatment of radians as dimensionless. There is a discussion on whether to interpret Ω as revolutions per second and how this relates to the period T.

Discussion Status

Several participants have provided calculations and expressed uncertainty about the treatment of radians in their unit analysis. Some guidance has been offered regarding the interpretation of radians as dimensionless, but no consensus has been reached on the overall understanding of the units involved.

Contextual Notes

There is an ongoing exploration of the implications of treating radians as dimensionless, and participants are encouraged to clarify their unit derivations. The original poster notes that the context is specifically related to gyroscopic precession, where gravity is the primary force causing torque.

lightlightsup
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Homework Statement
The precession rate is given as ##Ω = \frac{Mgr}{Iω}##.
Relevant Equations
What are the units here?: I'm calculating it as ##\frac{1}{rad . s}##.
The precession rate is given as ##Ω = \frac{Mgr}{Iω}##.
What are the units here?: I'm calculating it as ##\frac{1}{rad . s}##.
Am I supposed to interpret this as revolutions per second, sort of like frequency, and ignore the ##rad##?
Also, period is calculated as: ##T = \frac{2π}{Ω}##. So, ##T##'s units are ##\frac{s}{rev}##?
I'm guessing that I don't quite understand yet how ##rads## are ignored in the calculations.
Edit: This refers to gyroscopic precession wherein gravity is the only force causing a torque.
 
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lightlightsup said:
Homework Statement: The precession rate is given as ##Ω = \frac{Mgr}{Iω}##.
Homework Equations: What are the units here?: I'm calculating it as ##\frac{1}{rad . s}##.
I get 1/s, which would be the same as the units for ω...

EDIT -- can you show the units you have for the first equation, and what you cancel to get your result?
 
berkeman said:
I get 1/s, which would be the same as the units for ω...

We're ignoring ##rads##, I guess? Because they are considered "dimensionless" ratios?
 
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berkeman said:
I get 1/s, which would be the same as the units for ω...

EDIT -- can you show the units you have for the first equation, and what you cancel to get your result?

##Ω =\frac{Mgr}{Iω} = \frac{kg . \frac{m}{s^2} . m}{kg.m^2.\frac{rads}{s}} = \frac{1}{rad.s} ##
##T = \frac{2π}{Ω} = \frac{\frac{2π}{rad}}{\frac{1}{rad.s}} = s##

I'm sure there is some hole in my logic here somewhere.
 
Since radians are dimensionless, don't carry them along as units. In that case, you get the correct units for Omega, IMO.
 
lightlightsup said:
##Ω =\frac{Mgr}{Iω} = \frac{kg . \frac{m}{s^2} . m}{kg.m^2.\frac{rads}{s}} = \frac{1}{rad.s} ##
##T = \frac{2π}{Ω} = \frac{\frac{2π}{rad}}{\frac{1}{rad.s}} = s##

I'm sure there is some hole in my logic here somewhere.
Over the years there have been numerous attempts to assign a dimension to angles. You can find mine at https://www.physicsforums.com/insights/can-angles-assigned-dimension/
In respect of this thread, the interesting feature is that if we write the dimensionality as Θ then Θ2=1. So 1/rads is the same as rads.
 

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