Behaviour of Gyro in space compared to on Earth / in gravity


by mike walker
Tags: behaviour, compared, earth, gravity, gyro, space
mike walker
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#1
Nov16-12, 01:44 PM
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My question is about a toy gyroscope free floating in space far away from any gravitational field. I want to know why it does not precess in response to external force?

On earth the torque on a tabletop gyroscope comes from (i) the table pushing up, and (ii) the gyroscope's inertia resisting the push around it's centre of mass - causing spin. In space it's the same - except that instead of a table we use an astronaut's finger to push rather than gravity. Yet in space, the gyro will not precess as it does on Earth.

DETAILS:

What is the difference between applying 1g of tilting force to a gryoscope here on earth (gravity) and doing the same thing a gyroscope floating in space by pushing it with a finger.

Here on earth a tabletop gyroscope will precess due to the earth's gravitational pull applying a tilting force around the centre of mass. However, in space, a tilting force applied to one end of the axis does not precess the gyroscope. it will maintain its orientation while accelerating with the force without tipping, pitching or precession.

Why do they behave differently? The inputs are the same.

In both cases torque is applied to the gyroscope. On earth by gravity accelerating the table upwards (against freefall). If the force from the table is not aligned with the centre of mass of the gyroscope this will cause torque. In space the astronaut's finger pushes one end of gyroscope, if finger is not aligned with the gyroscopes centre of mass it will cause a torque.

So if the force and the resulting torque are the same in space and on earth why are the results so different? Why does the space gyroscope maintain orientation without precession?

In summary, can anyone explain why a free floating gyro in space does not precess in response to external force applied at one end of the axis.
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A.T.
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#2
Nov16-12, 02:06 PM
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On the table gravity and table continuously exert forces, which result in a sustained torque on a tilted gyro. When pushed around in space, the gyro floats away from the finger and the torque disappears. To have a comparable situation the astronaut would have to continuously accelerate the gyro at 1g, with an off center force.
BobG
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#3
Nov16-12, 02:19 PM
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A torque applied to a gyro in space has the exact same affect as a torque applied to a gyro on Earth.

If there's a difference, it has to be because the astronaut isn't actually applying a torque to the gyro, even though he's applying a force to it. I have a hard time imagining he wouldn't apply at least some torque to the gyro, though.

However, even if he is applying a torque, precession will only occur as long as the force is applied. It stops as soon as the gyro leaves the astronaut's finger.

On Earth, the force of gravity is constant, so you never see the gyro stop precessing.

mike walker
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#4
Nov16-12, 02:33 PM
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Behaviour of Gyro in space compared to on Earth / in gravity


Many thanks for clarifying that. A couple of articles I read said the gyro in space would not precess - that's what confused me.

For example I read this:

"In the weightless environment of the space shuttle, a spinning toy gyroscope was recorded on videotape. The gyro spun around an axis that kept pointing toward the same distant star. Even when an astronaut pushed on the gyroscope, it stubbornly maintained the orientation of its axis as it flew across the cabin. In the absence of twisting forces, a gyroscope 's axis will always point in whatever direction it was pointing when you started it spinning."

http://isaac.exploratorium.edu/~paul...gyroscopes.htm

That article says that the difference in space is that there are no twisting forces. It says that on earth there are two forces - the table pushing the axle up, and gravity pulling the gyroscope down - and this creates torque. Whereas in space there is only one force - so no torque. I dont have a physics background but I think that is nonsense. In space there is still torque if the astronaut maintains the force - i.e. accelerates.


From what you say, if the Astronaut maintained the force then the gyro would precess while the force is being applied. That makes a lot more sense so I pretty much understand it now.


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