Engineering / Physics Teaser Question on Gyroscopic Motion

M.D. Holloway · 05-17-2007, 08:18 AM

A physicist played a prank on his friend by placing a large spinning wheel inside a suitcase. He asked his friend to go to the next room and bring back the suitcase. His friend picked up the handle of the case in his left hand, unaware that the wheel was spinning inside in a counterclockwise motion, with the top of the wheel moving towards the front of the case. What happened to the case when his friend exited the room and made a left-hand turn?

oldE · 05-17-2007, 08:43 AM

He tripped over the case.

Les

Jim727 · 05-17-2007, 08:48 AM

Well, Mr. Wizard....

Assuming the axis of the gyroscope is horizontal and rigidly fixed to the inside of the suitcase, the suitcase would attempt to rotate clockwise wrt the guy carrying it. At least if I remember my right-hand-rule correctly.

Aerkuld · Aerkuld's Garage

If I'm thinking about this correctly the rotating flywheel would attempt to rotate clockwise along a horizontal axis when viewed from behind. The combined action of the rotation about the vertical axis and the flywheels reactive rotation would cause the top rear corner of the suitcase to move towards the carrier and the bottom front corner of the suitcase to move away, effectively causing a rotation around a roughly diagonal axis.

GO DAWG GO · GO DAWG GO's Garage

The gyroscopic effect would only stabilize the suitcase and maintain the heading or orientation of the wheel and limit translation.. If the carrier turned the suitcase would not eccelerate in the vector it would continue in its present processive course. That is if the rotational axis is horizontal.

If I understand your scenerio correctly.

Rick Lee · Rick Lee's Garage

If it's inside the case, it can't be seen and so Heisenberg's Uncertainty Princ. kicks in. Shining light on it (opening the case) would affect the movement. So there's no way to know to really know what happens. Sort of like a tree falling with no one around to hear it.

GO DAWG GO · GO DAWG GO's Garage

After reading the question again..I completely agree with oldE. He is absolutely right. even if the wheel was spinning the opposite direction. The suit case would trip the guy....

Aerkuld · Aerkuld's Garage

I think you have to assume that the person carrying the case does try to rotate it about it's vertical axis.
I'm thinking about the way you lean a motorcyle into a corner by turning the steering in the opposite direction to the turn of the corner.

cashflyer · 05-17-2007, 10:18 AM

Torque-induced gyroscopic precession will cause the case to pitch perpendicular to the rotational axis, about 90 degrees forward of the point of induced torque.

Aerkuld · Aerkuld's Garage

Quote:

Originally posted by cashflyer
Torque-induced gyroscopic precession will cause the case to pitch perpendicular to the rotational axis, about 90 degrees forward of the point of induced torque.

I agree. What your saying is that the spinning flywheel will produce a rotation at 90 degrees to the original input rotation and 90 degrees to the axis of rotation of the flywheel. But do you not then get a resultant rotation that is a combination of the intended rotation and the torque induced rotation?

GO DAWG GO · GO DAWG GO's Garage

Cashflyer, The rotational ecceleration are very limited....Your right if they are ~2+ G eccelerations. The component of torque would be negligable

THUS... Tripping the guy!

cashflyer · 05-17-2007, 10:35 AM

Hey... I only know helicopter blades, not suitcases. That requires a whole different field of study.

Aerkuld · Aerkuld's Garage

Quote:

Originally posted by Aerkuld
I agree. What your saying is that the spinning flywheel will produce a rotation at 90 degrees to the original input rotation and 90 degrees to the axis of rotation of the flywheel. But do you not then get a resultant rotation that is a combination of the intended rotation and the torque induced rotation?

To answer my own question - NO.

legion · 05-17-2007, 10:48 AM

Is this another thread where Lubey is never really going to give us the answer?

Porsche-O-Phile · Porsche-O-Phile's Garage

My guess is that the gyroscope did not stay rotating once the case was closed due to improper lubrication by an inferior grade of industrial lubricant.

M.D. Holloway · 05-17-2007, 01:47 PM

Ok, here goes...

The case pivoted top over bottom due to gyroscopic motion. The top of the case rotated sideways towards his friend's leg while the bottom of the case rotated upwards, away from him. Gyroscopic motion is fairly complicated, but can be explained by picturing the spinning wheel as a square ring with a heavy fluid flowing through it. As the friend turns left, the fluid flowing in a vertical (up or down) motion does not change direction. The fluid flowing horizontally, does change direction as the case is turned. A portion of the fluid that is flowing vertically (up or down) is forced to travel in a curve as the friend turns left. This exerts a force on the side of the case, forcing the bottom to lift away from the friend and the top to rotate towards him. The resulting motion of the rotating wheel is exactly the same as the fluid flowing through the square ring.

M.D. Holloway · 05-17-2007, 01:51 PM

Quote:

Originally posted by Porsche-O-Phile
My guess is that the gyroscope did not stay rotating once the case was closed due to improper lubrication by an inferior grade of industrial lubricant.

I always did like the way you thought!

Burnin' oil · 05-17-2007, 01:51 PM

Very small rocks?

Jim727 · 05-17-2007, 01:55 PM

Quote:

Originally posted by LubeMaster77
Ok, here goes...

The case pivoted top over bottom due to gyroscopic motion. The top of the case rotated sideways towards his friend's leg while the bottom of the case rotated upwards, away from him. Gyroscopic motion is fairly complicated, but can be explained by picturing the spinning wheel as a square ring with a heavy fluid flowing through it. As the friend turns left, the fluid flowing in a vertical (up or down) motion does not change direction. The fluid flowing horizontally, does change direction as the case is turned. A portion of the fluid that is flowing vertically (up or down) is forced to travel in a curve as the friend turns left. This exerts a force on the side of the case, forcing the bottom to lift away from the friend and the top to rotate towards him. The resulting motion of the rotating wheel is exactly the same as the fluid flowing through the square ring.

Clockwise. TaDaaaaaaa!

Hoots · 05-17-2007, 03:11 PM

Quote:

Originally posted by LubeMaster77
The case pivoted top over bottom due to gyroscopic motion. The top of the case rotated sideways towards his friend's leg while the bottom of the case rotated upwards, away from him. Gyroscopic motion is fairly complicated, but can be explained by picturing the spinning wheel as a square ring with a heavy fluid flowing through it. As the friend turns left, the fluid flowing in a vertical (up or down) motion does not change direction. The fluid flowing horizontally, does change direction as the case is turned. A portion of the fluid that is flowing vertically (up or down) is forced to travel in a curve as the friend turns left. This exerts a force on the side of the case, forcing the bottom to lift away from the friend and the top to rotate towards him. The resulting motion of the rotating wheel is exactly the same as the fluid flowing through the square ring.

You could have just said oldE was correct.