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# How does a gyroscope counteract the roll of a ship?

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1. Hi all. This is a question from the applied unit in Aqa physics.

Can anyone explain to me exactly how does a gyroscope work? From what I understand a flywheel store rotational energy when the ship rotates, but does that mean if the ship rotates clockwise, the conservation of momentum brings the ship back by rotating it anti-clockwise?

I know I might be horribly wrong as I really struggle with the content in the applied unit......
2. (Original post by Zechi)
Hi all. This is a question from the applied unit in Aqa physics.

Can anyone explain to me exactly how does a gyroscope work? From what I understand a flywheel store rotational energy when the ship rotates, but does that mean if the ship rotates clockwise, the conservation of momentum brings the ship back by rotating it anti-clockwise?

I know I might be horribly wrong as I really struggle with the content in the applied unit......
This is simply Newton's Law of inertia - that an object will continue moving in a straight line or in it's state of rest unless acted on by an external force.

The Gyroscope rotates in the same horizontal plane as the X-X axis.

i.e. all of the mass of the gyroscope is moving in that horizontal plane and according to Newton, will keep moving in the horizontal plane.

As the ship tries to change the gyroscopes' plane of rotation, the gyroscope mass will present a large inertia to the rolling motion of the ship. Action and reaction will then act to return the ship to it's original stable state of rest.
3. (Original post by uberteknik)
This is simply Newton's Law of inertia - that an object will continue moving in a straight line or in it's state of rest unless acted on by an external force.

The Gyroscope rotates in the same horizontal plane as the X-X axis.

i.e. all of the mass of the gyroscope is moving in that horizontal plane and according to Newton, will keep moving in the horizontal plane.

As the ship tries to change the gyroscopes' plane of rotation, the gyroscope mass will present a large inertia to the rolling motion of the ship. Action and reaction will then act to return the ship to it's original stable state of rest.
Thank you for your response. Is it ok if you can explain to me why does the gyroscope will present a large inertia? Is it because the gyroscope is further away from the axis of rotation?
4. I will post what the mark scheme says. It's a bit over simplified so I am having a hard time understanding this:
2marks for reference to conservation of momentum
1mark for 'external torque needed to change direction of flywheel axis'
1mark for 'hence reaction on the deck(counteracts roll)
5. (Original post by Zechi)
Thank you for your response. Is it ok if you can explain to me why does the gyroscope will present a large inertia? Is it because the gyroscope is further away from the axis of rotation?
The gyroscope is designed to have a flywheel with a large mass which rotates with high velocity. Inertia = mv. That mass rotates in the same x-x plane as the ships deck.

Visualise breaking the gyro into smaller 'spokes', radiating from the centre of rotation. Further, break each spoke into segments. Each segment of a spoke at any instant in time, will ever closer approximate to a straight line tangential motion.

Any rolling motion of the ship (torque) in the same x-x axis plane, produces an acceleration on each spoke segment, which tries to change the segments tangential straight line motion. i.e. change in inertia is the definition of force. Conservation of inertia will apply an equal and opposite reaction (torque) by each gyroscope flywheel segment, against the force trying to change the segment motion.

Sum these reactions for each segment and spoke and the total reaction force (torque) resisting the rolling motion of the ship soon becomes large. The rolling motion of the ship is thus stabilised / damped.
6. (Original post by uberteknik)
The gyroscope is designed to have a flywheel with a large mass which rotates with high velocity. Inertia = mv. That mass rotates in the same x-x plane as the ships deck.

...

Any rolling motion of the ship (torque) in the same x-x axis plane, produces an acceleration on each spoke segment, which tries to change the segments tangential straight line motion. i.e. change in inertia is the definition of force. Conservation of inertia will apply an equal and opposite reaction (torque) by each gyroscope flywheel segment, against the force trying to change the segment motion.
I don't like to be overly critical but this explanation is borderline gobbledygook.

1. "Inertia = mv."

mv is momentum; inertia is the property of mass that it can't be accelerated in the absence of a force

2. "change in inertia is the definition of force"

The force acting on a body is numerically equal to the rate of change of a body's momentum

3. "Conservation of inertia.."

There is no such principle; we have conservation of momentum and angular momentum for isolated systems, but not conservation of inertia.

This problem really concerns the fact that if a system has angular momentum (which is the case for a gyroscope), then the angular momentum can only be changed if a torque is applied to the body.

If you want to rotate the axis of rotation of a gyroscope with angular momentum at a rate , you need to apply a torque of size to it.

Since a torque is merely a force acting to turn a body, Newton III applies, and the body applying the torque feels a Newton III torque in the opposite sense.

In this case, the boat is applying a torque to a gyroscope with large L (it's spinning fast, and has lots of mass), so it must apply a large torque to rotate its axis of rotation at a given rate. (Note that the gyro is attached rigidly to the boat so if the boat tries to rotate, then so does the gyro.) The boat then feels an identical torque from the gyro in the opposite sense i.e. opposite to the way the external world (i.e. the sea or wind) is trying to turn it.

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