Question about Rotation

[SilentSky23]

This should be in relation to rotational physics. Now, say you have a rotating person. Would it be possible to change axis of rotation/change direction of rotation while rotating? For example, say the person is doing a head-to-toe twist, but goes from that to a cartwheel or a flipping rotation. Would that be possible? Why or why not?

[zapatos]

Do you mean like Olympic divers do?

[SilentSky23]

4 minutes ago, zapatos said:

Do you mean like Olympic divers do?

Depends. Do they go from twisting motions to other forms of rotations such as flips/somersaults and/or cartwheel like motions?

[studiot]

6 hours ago, SilentSky23 said:

This should be in relation to rotational physics. Now, say you have a rotating person. Would it be possible to change axis of rotation/change direction of rotation while rotating? For example, say the person is doing a head-to-toe twist, but goes from that to a cartwheel or a flipping rotation. Would that be possible? Why or why not?

We need to work through your questions logically.

 

People are three dimensional and therfore have three rotational degrees of freedom.

This means that three simultaneous separate rotations about the three coordinate axes are possible.

I do not know of people executing three separate rotations, but skaters have a manoeuvere involving two.

When the skater is spinning about a vertical axis, balanced on one leg, the skater leans forwards and pokes her free leg backwards.
Then she flips supporting legs by swinging the free leg in a wide arc to land on it.
This is then rotation about a second horizontal axis form head to toe through the skater.

It is well to enquire how the rotation starts in the first place.

This second rotation is started in the same way as the first by bodily swinging a fairly heavy part of the body.

A further wrinkle is the possibility of employing the gyroscopic force.
This is a force on a spinning object that is also moving along a (straight) line, at right angles to that line.

 

I do not know of any manoeuvere where people reverse the direction of rotation, but the balance wheel in clocks does exactly that, due to the action of its mechanism.

 

[swansont]

With a rigid body, no. You need an external torque to change the angular momentum. With a non-rigid body, I think it's possible that one might be able to contort and change one's orientation to realign along a new axis. i.e. the rotation would remain along the same coordinate axis, but the body's orientation has changed.

[SilentSky23]

So it is possible. Can anyone explain how and why it is possible, in more detail if it has been done already?

[studiot]

11 hours ago, SilentSky23 said:

So it is possible. Can anyone explain how and why it is possible, in more detail if it has been done already?

Swansont has said when it is not possible and I have outlined circumstances when it is possible.

 

What did you not understand?

[swansont]

11 hours ago, SilentSky23 said:

So it is possible. Can anyone explain how and why it is possible, in more detail if it has been done already?

A human simply jumping under his/her own power is probably not aloft long enough to do this. But as we probably all know, a cat can contort itself to land on its feet, with no angular velocity. A human can launch itself off a springboard, head up, and enter the water head down, again with no angular velocity.

That can happen because we have internal motions and can change our various moments of inertia. e.g. if you twist your torso while your arms are extended, you have a large moment of inertia, so your legs have to rotate a lot more in the opposite direction to compensate. Then you can bring your arms back in and undo the twist. You will have rotated along your long axis with no external torque. (this could be be done while jumping vertically on a trampoline) Cats do similar things with their legs while falling to get their feet pointed down.

[SilentSky23]

Okay, I think I get it somewhat. Still, say a person is spinning in the X axis, but wants to change their axis of rotation to the Y axis or Z axis while still spinning at full speed without slowing down or stopping to do so. Would internal motions of the human body allow for that as well?

[studiot]

1 hour ago, SilentSky23 said:

Okay, I think I get it somewhat. Still, say a person is spinning in the X axis, but wants to change their axis of rotation to the Y axis or Z axis while still spinning at full speed without slowing down or stopping to do so. Would internal motions of the human body allow for that as well?

Don't want much do you?

And what on earth do you mean by spinning in the x axis?

If they are spinning about the x axis, the x coordinate is the one that will not change.

[SilentSky23]

3 minutes ago, studiot said:

Don't want much do you?

And what on earth do you mean by spinning in the x axis?

If they are spinning about the x axis, the x coordinate is the one that will not change.

And you mean what by the last part? Does it have to do with rotation?

Let me try a better example.

As show here, there are three axis of rotation for a plane, the Yaw axis, which is twisting like, the Roll Axis, which is cartwheeling like, and the Pitch axis, which is flipping like. Now, let us not apply these rotations to the plane, but to a body like the human body. The say the human body rotates around the "yaw axis", but then suddenly changes to spinning in the roll axis or pitch axis, without slowing down or stopping. How hard would that be if it were possible, if it is possible at all?

And by the X, Y and Z axes, I meant this.

[swansont]

6 hours ago, SilentSky23 said:

Okay, I think I get it somewhat. Still, say a person is spinning in the X axis, but wants to change their axis of rotation to the Y axis or Z axis while still spinning at full speed without slowing down or stopping to do so. Would internal motions of the human body allow for that as well?

No. Angular momentum is conserved. What they might be able to do is re-orient themselves so that they are aligned along a different axis. But the angular momentum about the x-axis will not change.

[SilentSky23]

28 minutes ago, swansont said:

No. Angular momentum is conserved. What they might be able to do is re-orient themselves so that they are aligned along a different axis. But the angular momentum about the x-axis will not change.

One more thing, why is angular momentum conserved?

EDIT: And by realigning along a different axis, you mean by tilting while cartwheeling or flipping so you are cartwheeling or flipping or twisting 90 degrees or so in another axis/angle?

Edited January 23, 2018 by SilentSky23

[swansont]

8 hours ago, SilentSky23 said:

One more thing, why is angular momentum conserved?

T = dL/dt

Torque is the time rate of change of angular momentum. If there is no external torque, angular momentum must remain constant.

8 hours ago, SilentSky23 said:

EDIT: And by realigning along a different axis, you mean by tilting while cartwheeling or flipping so you are cartwheeling or flipping or twisting 90 degrees or so in another axis/angle?

If you were tumbling (e.g. a somersault), I mean twisting your body so that you would be cartwheeling or spinning like a skater. The axis of rotation would not change relative to fixed space.

[studiot]

The difference between Swansont's examples and mine are that his examples are of divers or acrobats in free air, with no source of external torque (apart from the initial pus off as they jump).

My skater is always in contact with something to push against.

This is why again I say to you,  " Your scenario is too vague. Please narrow the circumstances down".

 

Edit.  Note your aircraft is always pushing against the air.

Edited January 23, 2018 by studiot

[geordief]

1 hour ago, studiot said:

The difference between Swansont's examples and mine are that his examples are of divers or acrobats in free air, with no source of external torque (apart from the initial pus off as they jump).

My skater is always in contact with something to push against.

This is why again I say to you,  " Your scenario is too vague. Please narrow the circumstances down".

 

Edit.  Note your aircraft is always pushing against the air.

Is this a nice illustrative picture?

 

 

If the man is first set to spin  (by external force ) on a N/S  axis ,followed by the E/W axis  and then the axis perpendicular to the screen (if the man is straightened up at the outset  to be looking directly out at the viewer) do we have 3 simultaneous rotations?

 

Do the 3 rotational effects add linearly? Can they be described as one overall rotation or are they separate?

[studiot]

1 hour ago, geordief said:

 

 

Do the 3 rotational effects add linearly? Can they be described as one overall rotation or are they separate?

Not even for two rotations, let alone 3.

You cannot combine vectorially since AB =/= BA, if A and B are rotations.

http://demonstrations.wolfram.com/CombiningTwo3DRotations/

You need quaternion or matrix algebra to find the result.

 

[Bender]

You could start out with arms stretched and legs curled up, so you have a fast pitch but a slow roll. Mid air, you can stretch your legs and bring your arms together. This will slow down your pitch and speed up the roll.

When acrobats or divers do this, the slower rotation can be slow enough compared to the short air time that they appear to have stopped.

That way, a diver can appear to be Rolling during the first half of a dive and pitching during the second while the angular momentum remains constant. 

Edited January 23, 2018 by Bender

[SilentSky23]

17 hours ago, swansont said:

If you were tumbling (e.g. a somersault), I mean twisting your body so that you would be cartwheeling or spinning like a skater. The axis of rotation would not change relative to fixed space.

Could you explain that a bit more, please?

[J.C.MacSwell]

11 hours ago, SilentSky23 said:

Could you explain that a bit more, please?

Assumptions being no external forces and you jettison nothing (no cheating by spitting etc and you are isolated in space):

You can reorient yourself with respect to your axis of rotation which is fixed in your inertial frame, always about your center of mass which is also fixed in that frame. You can do this by waving arms about, contorting etc...or if you are a "little eccentric" you can do it with no effort...

Note in the video how the axis of rotation remains fixed

 

Edited January 24, 2018 by J.C.MacSwell

[SilentSky23]

8 hours ago, J.C.MacSwell said:

Assumptions being no external forces and you jettison nothing (no cheating by spitting etc and you are isolated in space):

You can reorient yourself with respect to your axis of rotation which is fixed in your inertial frame, always about your center of mass which is also fixed in that frame. You can do this by waving arms about, contorting etc...or if you are a "little eccentric" you can do it with no effort...

Note in the video how the axis of rotation remains fixed

 

So, you can reorient yourself to spin on the X axis while positioned on the Z or Y Axis (upside down or lying on the side) while still spinning as if on the X axis?

[J.C.MacSwell]

1 hour ago, SilentSky23 said:

So, you can reorient yourself to spin on the X axis while positioned on the Z or Y Axis (upside down or lying on the side) while still spinning as if on the X axis?

Using a frame with fixed axes in space, you remain spinning on the same axis the whole time. Using your example with the aeroplane (which has a commonly defined frame that is not generally inertial) say you had something massive inside that could contort like a cat, and you were isolated in space. You could realign the plane so that the axis of spin was along any alignment wrt the aeroplane. Most alignments would not be stable and they would tend to misalign, but at all times the axis of spin would remain fixed in space (it might not seem it at times, and the massive "cat" on board would have to be included)

I hope that makes sense. Doesn't seem as clear as I hoped it would sound.

Edited January 25, 2018 by J.C.MacSwell

[SilentSky23]

57 minutes ago, J.C.MacSwell said:

Using a frame with fixed axes in space, you remain spinning on the same axis the whole time. Using your example with the aeroplane (which has a commonly defined frame that is not generally inertial) say you had something massive inside that could contort like a cat, and you were isolated in space. You could realign the plane so that the axis of spin was along any alignment wrt the aeroplane. Most alignments would not be stable and they would tend to misalign, but at all times the axis of spin would remain fixed in space (it might not seem it at times, and the massive "cat" on board would have to be included)

I hope that makes sense. Doesn't seem as clear as I hoped it would sound.

I think I got it, but a simple yes or no, or not exactly would have sufficed.

[Mordred]

On 24/01/2018 at 6:12 PM, SilentSky23 said:

I think I got it, but a simple yes or no, or not exactly would have sufficed.

Where is the fun of learning in that type of answer 

[SilentSky23]

20 hours ago, Mordred said:

Where is the fun of learning in that type of answer 

If you say so. Thanks for pointing that out, though. It was good to learn that.