Why Does Perpendicular Torque Not Change Angular Momentum Magnitude?

[Kenny Lee]

Hi! Simple question I've got to ask.

The cause of precession is the torque due to weight, causing a change in angular momentum. It's stated in my textbook that the angular momentum's magnitude remains unchanged because the torque, and hence, dL, is perpendicular to the angular momentum's direction.
However, it doesn't explain why perpendicular summation of L and dL results in zero change of L's magnitude. If anyone could help clarify, thanks a lot.
Lemme know if my Q doesn't make sense. I'll try rewording if that is the case! Chill.

 

[Chi Meson]

Hi,
The proper explanation of this phenomenon is best saved for when you can do 3-D vector matrices, or similar calculus. Until then, you could read a conceptual description I made a few months ago. Unfortunately, the guy I responded to then turned out to be a banned crackpot, so I'm glad that the explanation has a second chance to not be a "waste of time."

see here:
https://www.physicsforums.com/showthread.php?t=74561

 

[Kenny Lee]

That complicated? And to think I was hoping for something light and easy to munch on before sleep. Anyway, thanks for advice!

 

[Chi Meson]

A simple description would require drawing diagrams and I just can't do it well on the computer. Although 3-D CG graphics do a great job at demonstrating the cause of precession. Try googling.

 

[Doc Al]

It's stated in my textbook that the angular momentum's magnitude remains unchanged because the torque, and hence, dL, is perpendicular to the angular momentum's direction.
However, it doesn't explain why perpendicular summation of L and dL results in zero change of L's magnitude.

The same mathematical situation occurs in uniform circular motion: the acceleration is towards the center, thus dv is always perpendicular to the velocity (which is tangent to the circle). The speed never changes, just the direction.

But please do study Chi Meson's conceptual description of precession; you'll learn something.