# Gyroscope precession - WHY? (What's the cause?)

1. Feb 25, 2006

### panathi

«(...) a torque τ applied perpendicular to the axis of rotation, and therefore perpendicular to L, results in a motion perpendicular to both τ and L. This motion is called precession.»(http://en.wikipedia.org/wiki/Gyroscope)

Why does this happen? Assumpting this makes all further calculus very easy and we can calculate the angular speed of the precession motion without difficulty (see http://physics.nad.ru/Physics/English/gyro_txt.htm).

But WHY do we assumpt this? What's the reason why the gyroscope does not rotate over the "expected" axle, as it does when the rotor is stopped? (I do not want you to tell me about experimental facts... I think it is pretty obvious that I am seeking for a theorical explanation)

Stay cool!

2. Feb 25, 2006

### pervect

Staff Emeritus
Because the angular momentum of the gyroscope is conserved.

Draw the vector L for the initial angular momentum of the gyroscope.

Draw the vector $\Delta T \tau$ for the amount of angular momentum transfered to the gyroscope from a torque $\tau$ acting for a time $\Delta T$.

Observe that the final angular momentum of the gyroscope is given by the intial angular momentum plus torque*time.

3. Feb 26, 2006

### Tide

First, it is not an assumption! It follows quite rigorously from Newton's Laws of Motion applied to a rotating system or rigid body. As for the "expected" behavior, it is only "expected" if you insist on thinking about it as a nonrotating system.

You can find an adequately detailed explanation here: http://en.wikipedia.org/wiki/Precession

4. Feb 26, 2006

### Staff: Mentor

This is easier if you analyze a simple hypothetical object such as two equal point masses attached to the ends of a massless rigid rod, that rotates around the midpoint of the rod. A long time ago, I saw a derivation that used this model, but I don't remember where.

5. Feb 26, 2006

### panathi

Thank you guys :)

(http://en.wikipedia.org/wiki/Precession)

This piece of information clears my doubts and explains why the gyroscope makes de precession motion. But why does the body tend to rotate around a line which is a principal axis of maximum moment of inertia?

Thank you all that answered me promptly. :)

Farewell

6. Feb 27, 2006

### SockCymbal

http://www.thehowandwhy.com/Gyroscopic.html [Broken]

Last edited by a moderator: May 2, 2017
7. Feb 28, 2006

### panathi

How do you define this?

8. Feb 28, 2006

### SockCymbal

Its pretty self explanatory. Its simply the axis that yeilds the highest (most resistivity to spin) moment of inertia.

The MINIMUM moment of inertia about any axis passes through the centriod (center of mass of a uniformly dense object).

9. Mar 1, 2006

### panathi

Can the axis who passes through the centriod be considered the principal axis of maximum moment of inertia? This makes no sense for me. :s Please keep helping please.