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17 February 2019 12:55

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Question

Asked by: Mike Walker
Subject: Gyroscope in space
Question: SUMMARY:
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?

Here on earth, I understand 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. Yes in space, the gyro will not precess.

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.
Date: 16 November 2012
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Answers (Ordered by Date)


Answer: Kysen Palmer - 16/11/2012 21:10:27
 Hi MIke,

In giving your question some thought, I believe the simplest answer is that there must be a coupled set of forces in order to create a torque. It is not enough to push on one end which is away from the center of mass. Without a coupled force, there is nothing to cause tilt instead of linear motion.

In more detail this means that the table friction on earth is the difference. The table friction acts in a direction opposite of the attempted motion such as that by the force applied. This causes there to be an overall torque applied causing procession.

In space, there is little friction, and because of this the gyroscope does not see a torque unless there is a second force preventing linear motion. If you would like a more detailed explanation let me know and I can attempt to a more in depth discussion of the mechanics.

Hope I helped. Cheers,

Kysen

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Answer: Kysen Palmer - 16/11/2012 21:10:47
 Hi MIke,

In giving your question some thought, I believe the simplest answer is that there must be a coupled set of forces in order to create a torque. It is not enough to push on one end which is away from the center of mass. Without a coupled force, there is nothing to cause tilt instead of linear motion.

In more detail this means that the table friction on earth is the difference. The table friction acts in a direction opposite of the attempted motion such as that by the force applied. This causes there to be an overall torque applied causing procession.

In space, there is little friction, and because of this the gyroscope does not see a torque unless there is a second force preventing linear motion. If you would like a more detailed explanation let me know and I can attempt to a more in depth discussion of the mechanics.

Hope I helped. Cheers,

Kysen

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Answer: Mike Walker - 16/11/2012 22:34:32
 Thank you for your the answer Kysen.

So " table friction on earth is the difference. The table friction acts in a direction opposite of the attempted motion such as that by the force applied. This causes there to be an overall torque applied causing procession".

I can extrapolate this to the vertical bicycle wheel gyro example - the pivot is fixed - so the wheel precesses. Thanks for pointing that out.

The bit I still do not understand is why no torque is applied in space - if for example an external force is applied to the gyro at a distance from its centre of mass.

As I understand it (correct me if am wrong) : On a table there is only one force pushing up - gravity which causes the table to accelerate upwards (GR) ; and one force resisting - the inertia of the gyroscope which wishes to be in freefall on earth. This creates torque.

As I see it the situation in space is the same - only one force pushing ( the astronauts finger) and one resisting (the inertia of the gyroscope at its centre of mass). This must also create torque (as I understand it).

I do not understand how it can be said that the astronauts finger will not create torque around the centre of mass of the free floating gyroscope. Can you clarify that bit please?

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Answer: Blaze - 18/12/2012 00:34:30
 Hi Mike. I believe you are correct. If a force is applied at a distance from the center of mass of the gyroscope in space, one "end" of the force couple is the inertial resistance of the gyro's mass and other "end" of the force couple is the force applied by the astronauts finger. So, since there is a force couple acting on the gyro it will precess. The gyro may also have some linear movement and would probably end up against a surface inside the spacecraft somewhere eventually if the astronaut kept applying the force to the gyro. The force supplied by the astronaut would be acting against the side of the spacecraft that the astronaut is bracing against, which would move the gyro towards the opposite interior surface.

regards,
Blaze

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