Why Does Perpendicular Torque Not Change Angular Momentum Magnitude?

In summary, the cause of precession is the torque due to weight, which changes the angular momentum. This change in angular momentum's magnitude results in a change in the direction of the orbiting body.
  • #1
Kenny Lee
76
0
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.
 
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  • #2
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
 
  • #3
That complicated? And to think I was hoping for something light and easy to munch on before sleep. Anyway, thanks for advice!
 
  • #4
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.
 
  • #5
Kenny Lee said:
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.
 

FAQ: Why Does Perpendicular Torque Not Change Angular Momentum Magnitude?

What is a spinning top?

A spinning top is a simple toy that consists of a cone-shaped body with a pointed tip on one end and a flat base on the other. It is typically made of wood or plastic and has a metal or plastic rod through its center, which allows it to spin on a flat surface.

How does a spinning top work?

A spinning top works by converting the force of gravity into angular momentum. When the top is spun, the force of gravity pulls down on the top, causing it to rotate around its central axis. This rotation creates angular momentum, which keeps the top stable and spinning for a longer period of time.

What is angular momentum?

Angular momentum is a physical property of a spinning object that describes its tendency to resist changes in its rotational motion. It is a combination of an object's mass, velocity, and distribution of mass around its axis of rotation.

How is angular momentum related to a spinning top?

Angular momentum is essential for a spinning top to maintain its balance and continue spinning. As the top spins, its angular momentum keeps it upright and prevents it from falling over. The faster the top spins, the greater its angular momentum, and the longer it will continue to spin.

What factors affect the angular momentum of a spinning top?

The angular momentum of a spinning top is affected by several factors, including its mass, shape, and speed of rotation. The distribution of mass in the top also plays a significant role in its angular momentum, as a top with more mass concentrated towards the tip will spin longer than a top with more mass towards the base.

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