Angular momentum
Question:

 

A rotating rigid body have an angular momentum underset{L}{
ightarrow} which is equal to the product of Iunderset{omega }{
ightarrow} where Iis the moment of inertia and underset{omega }{
ightarrow} is the angular velocity.
If a rigid-body spins along its main Axes it will continue spinning unless there is a torque  underset{	au }{
ightarrow} acts on it.
A torque equals to the vector product of underset{F}{
ightarrow} x underset{r}{
ightarrow} where  underset{F}{
ightarrow} is the force and underset{r}{
ightarrow}is the distance from the rotation axis and its direction is acording to Right - hand grip rule.

When a torque underset{	au }{
ightarrow} acts on a rotating rigid body it will change the value and the direction of the angular momentum, underset{Delta L}{
ightarrow}/Delta t=underset{	au }{
ightarrow}  (see animation 1)

For more information see Blossoms lesson: https://blossoms.mit.edu/videos/lessons/physics_boomerangs

In this question we can neglect friction.

A disk that has a mass M and a moment of inertia I rotates fast with an angular velocity of  underset{omega }{<br>ightarrow}​ around a rod with a negligible mass along the x axis - see vedio  illustration.

How do you think the disk motion will be?

1 People tried to answer this question

The weight of the disk will cause it fall down.

The angular momentum will cause the disk to stay at the same place until it stopsand fall dawn.

The angular momentum of the rotating disk will resist changing in its direction so the disk have to continue rotating at the same place.

The disk weight will cause a torque that will change the angular momentum and will cause an addition rotation (precession) around the y axis.

Fantastic :-)

Wrong answer:-(think about the torque caused by Mg.

Wrong answer:-(think about the torque caused by Mg.

Wrong answer:-(remember there is no friction



Comments:
1.  Ismael Dalal (26.06.2017 am 03.31.25)
Peer evaluation

Dear Ibrahim Rami,

The scientific explanation which is present before the video is of high quality since it explains a very complex subject in a good and invested way, unfortunately, because I don't come from physics field, I couldn't understand the theoretical material and the question clearly, perhaps you should explain it in a more simplified way to fit everyone, but certainly the question and the video on the high end, but there is still place for improvement.

Dalal



History of edits
Edited BY: rami ibrahim Edit Date: 2017-06-12 17:58:59
Edited BY: rami ibrahim Edit Date: 2017-05-23 01:25:38
Edited BY: rami ibrahim Edit Date: 2017-05-23 01:23:55
Edited BY: rami ibrahim Edit Date: 2017-05-23 01:22:13
Edited BY: rami ibrahim Edit Date: 2017-05-23 01:20:18
Edited BY: rami ibrahim Edit Date: 2017-05-23 01:17:38