# Rotating body viewed from its own rotating ref. frame, then perturbed

• Swamp Thing
In summary, the thrusters may cause the disk to rotate, but this rotation can be ignored if the system is treated as a gyroscope.

#### Swamp Thing

Pondering over this thought experiment, a question comes to mind -- to which my brain sometimes replies "of course" and sometimes "no way!"A disk shaped satellite ("in zero-G") spins about its axis. There are two thrusters mounted as shown on two axial booms. The thrusters are fired briefly at some moment. We want to try and predict the effect of the thrusters while looking at the system as a co-revolving observer (e.g. we're strapped into a seat on a huge ring or shell that surrounds the disk symmetrically, and is initially rotating at the same angular velocity).

In other words, we want to predict the behavior of the disk, with respect to a rotating reference frame that the disk was stationary in before the thrusters were fired -- and this frame is not affected by the thrusters.

The question is, do we need to consider the disk to be a gyroscope -- and hence, do we need to grapple with some complicated precession stuff?
OR... can we ignore the rotation and just consider the disk as a simple moment of inertia that is acted upon by a torque?

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Swamp Thing said:
A disk shaped satellite ("in zero-G") spins about its axis. There are two thrusters mounted as shown on two axial booms.
Are the thrusters initially spinning with the disc or are they fixed in the inertial frame?

Swamp Thing said:
. can we ignore the rotation and just consider the disk as a simple moment of inertia that is
The disc is initially not rotating in your frame. But your frame is, so you have to consider inertial forces (centrifugal and Coriolis).

If the aim is understanding a gyroscope, the below video might help. You could for starters place the thrusters directly on the disc, such they the produce the same torque.

The thrusters are fixed to the axis rods, so they are fixed parts of the system.

Yes, I guess fixing them to the periphery would make it less complex looking and would be pretty much equivalent so I'll go with that.

I'll check out the video... Thanks.