Conservation of angular momentum

[trevorjohnson83]

when something like a spacecraft begins spinning in space, does the spin increase? decrease over time? maintain constant?

ChatGPT said:​

Good question — the answer depends on whether any external forces or torques act on it. Let’s look at the physical reasoning clearly:

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If it’s truly isolated in space:​

If the spacecraft is far from other bodies, with no engines firing, no thrusters, no drag, and no internal moving parts shifting angular momentum, then:

• Its spin will remain constant.
  This follows from conservation of angular momentum — one of the most robust laws in physics.

L=Iω=constantL = I \omega = \text{constant}L=Iω=constant
where
LLL = angular momentum,
III = moment of inertia,
ω\omegaω = angular velocity.
If III doesn’t change (no parts moving in or out), then ω\omegaω (spin rate) stays constant forever.

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However, in reality, a few subtle effects can change it over time:​

Even in space, there are tiny torques that can gradually alter spin:

1. Solar radiation pressure (YORP effect):
   Sunlight hitting and reflecting off the craft exerts small forces. Over months or years, this can either spin it up or slow it down.
2. Outgassing:
   If gases slowly leak or sublimate from the surface, they produce a tiny reactive thrust.
3. Magnetic and gravitational torques:
   • If the spacecraft has magnetic materials, they can interact with planetary magnetic fields.
   • Non-uniform gravity fields (if near a planet) can cause tidal torques that align and slow the spin (like how Earth’s gravity slowed the Moon’s rotation).
4. Internal moving parts:
   Rotating machinery, fuel slosh, or momentum wheels inside can transfer angular momentum, changing the overall spin rate or axis.

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In summary:​

Condition	Spin Behavior
Perfect isolation (no torque)	Constant spin forever
Small external torques	Gradual change (usually slows or drifts)
Controlled torque (thrusters or gyros)	Can be intentionally increased or decreased

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Would you like me to show how to calculate how long it would take for sunlight to noticeably slow or speed up a spinning spacecraft (using a simple example)?

 

[Darkwind]

Yes. The third law of motion is in play. But the third law of motion predicates that it is in a perfect vacuum.

An object at rest remains at rest, unless acted upon by outside forces. An object in motion will stay in motion unless acted upon by an outside force.

But it presupposes a perfect vacuum, which isn't possible even in intergalactic space. It gets close.

 

[scruffy]

A field qualifies as an "external force or torque".