Axes of rotation

[NetSquirrel]

About how many axes (not the chopping kind) can you rotate an object in a gravity- and air-free space, simultaneously? I'm finding this difficult to visualise. Obviously you can take the object and give it a bit of spin about an axis. From then on, can you apply a force that will make it keep spinning, but also spin about a second plane (i.e. make the original axis rotate)? I don't know whether this would just be prevented by the gyroscope effect or not. From then on, can you add more axes of rotation...what's the limit? <BR><BR> I've tried to draw out what I mean (see link) but it looks strange due to Paint Shop Pro not having 3D drawing tools. <BR><BR> http://images.dpchallenge.com/images_portfolio/16982/med ium/113918.jpg

[NetSquirrel]

...Also what is the thing we are supposed to do nowadays instead of < BR >?

[Drewhound]

One, try it with a ball. If you alter the axis it's still only one

[jenky]

I agree with Drewhound. A rotating body can only have 1 axis of rotation. If you have a rotating ball, then apply a force to make a spin on another axis, it can no longer be spinning on the original axis as well.

[jenky]

Having said that... if the body is an irregular shape (not a sphere) then this would not apply.

[New Forester]

I don't think it makes any difference what the shape actually is, if it's a rigid one.

 

I think Drewhound and Jenky must be right, but I too am having trouble visualising it.

 

If I am driving along in a straight line in a car, each of my front wheels is rotating about its axle.  If I then drive in a curve, the whole wheel is also rotating about the centre of the curve.

 

So where is the axis of rotation now?  It presumably can't still be the axle itself, but another "virtual" axis which is the resolution of the two rotations.  How do the bearings then still keep turning smoothly with an unaligned axis of rotation?

 

[jenky]

I think the original question (from the drawing), is referring to objects spinning on a axis within itself, rather than orbit like rotation which is where my working comes from. If we're talking about the earth for example, then it spins on it axis, but obviously also rotates about another point. I don't think this is what NetSquirrel meant though.

[Ralph]

It must be all 3 rotational axes of freedom at the same time. Has anyone ever seen those 80's disco light balls, the ones with loads of lights on the surface. You spin the centre sphere with the lights up around its axis which is horizontal through the centre. Then you spin up the sphere and its supporting structure about another axis which is vertically through the centre of the sphere. Now you've got the centre sphere spinning about two axes. Only one left.................... I'm thinking.

[Ralph]

Arghhhh! It's wrecking my head, mainly cos the third axis in the model I've suggested involves the nightclub spinning wildly around a disco ball! Excellent question.

[helghast]

an object can only rotate in one direction. if you try to do it in two it will actually rotate diagonally. this has been doing my brain in to

[DrWu]

I think an object can definitely rotate about two axis. Imagine a pen rotating about its length or horizontal axis (you know, so it doesn't appear to be moving). Then you can imagine it rotating through 360 degrees so that the ball and the bottom tip over each other still whilst rotating along its own length (you can try this at home kids!!) If anyone remembers the shuttle docking with the space station in 2001 A Space Odessy, it's the same idea I think. Having said all that if you consider time to be a dimension then it gets REALLY interesting!

[New Forester]

I still think there can only be one axis of rotation -- as Deamo says, the two will resolve as one diagonal one.

 

However, I don't think it makes any difference whether the axis is within or outside the object -- rotating about an external point is only adding an on-the-spot rotation and a translation-in-a-curve.

 

Still can't visualise it though.  I think it works out like vectors -- if you push something north and push it east at the same time it ends up going diagonally north-east -- it can't go two separate ways at once.

[Drewhound]

I think this is the important bit of the question

"Obviously you can take the object and give it a bit of spin about an axis. From then on, can you apply a force that will make it keep spinning, but also spin about a second plane (i.e. make the original axis rotate"

If you were to put two pins on opposite sides of a ball (though the shape isn't important) you could spin the ball around that axis. If you then have two other pins placed elsewhere and try to spin the ball using them as an axis it isn't going to work. Not very scientific but I can visualise this

 

 

[Drewhound]

Reading my own answer I'm still not sure. You could put the ball in a gimbals like arrangement and spin the whole thing in a different plane. But part of the question is "can you apply a force that will make it keep spinning, but also spin about a second plane (i.e. make the original axis rotate)?"

    I don't think you can apply a single force that would achieve both the motions

[Normski]

Imagine you had a big cermaic ball, which was hollow and their was a cat inside. You spin the ball around is longest horizontal axis relative to the floor (i.e spin it round on the spot currently touching the floor)

The cat, inside, climbs upwards thus altering the vertical axis' movement.

 

Until the friction of the contact with the floor makes the horizontal spin slow to a stop, then if the cat continually climbs and slips and repeats then the ball would be rotating on two axis'.

Now imagine that you and the ball are both on the inside of a larger ceramic ball, which is pinning along some other diagonal axis for example, then the original ball with the cat inside is actually spinng  in 3 directions.

 

Add more outer balls to add more axis.

Does that make sense ?

[BenDToy]

I believe the earth actually rotates about at least 2 axis, as well as orbiting the sun. It spins about the poles once per day, and also tilts. To visualise how this could be, imagine a globe, in the traditional style, with rods at each of the poles, top & bottom. It is easy to spin the globe while you hold the rods. Then turn the globe upside down by swopping the position of your hands. Works for me anyway!!

[Drewhound]

BenDToy, to start with I saw the answer clearly and then confused myself but if you look at NetSquirrel's diagram it's clear it can only rotate about one axis as it's shown

I'm not disputing what you say (I follow that) but I don't think it's what he meant

[MORELLO]

Three. As in a tri-plane axis such as the subtalar joint which sits beneath the ankle joint. This joint moves in the frontal, saggital and transverse planes as it creates elements of inversion/eversion, dorsiflexion/plantarflexion and supination/pronation simutaneously.

[helghast]

what in the hell are u going on about CJH?

u can rotate it an infinite number of ways, but only in one way at a time as it would be going in a diagonal direction if you tried to go more than one way 

[Ralph]

I did some thinking and modelling. As has been said, a single force will cause the body to rotate in one axis only. If you want it to rotate in all 3, you need to apply 3 different rotational forces (have 3 independent motors). What I ended up drawing is very like those fairground rides I think I've seen which comprises of three concentric rings with a seat in the middle. I did a little animation also, but I can't get it off the CD!

[NetSquirrel]

Thanks for the great answers! Howstuffworks.com has some good information to offer (http://science.howstuffworks.com/gyroscope1.htm and specifically http://static.howstuffworks.com/mpeg/gyro.mpg). The bike wheel spins about 2 axes in response to gravity constantly tugging at its "main" axis. Do you think, if gravity were suddenly removed, it would keep its second spin, or lose it (the kinetic energy of the second spin being transferred to the first spin, speeding it up)? I suspect one spin is the maximum, but there might be nothing else for it but to ask an actual astronaut to go into space test this out�