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Sphere spins faster at poles/axis?

by KidWonder
Tags: faster, poles or axis, sphere, spins
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KidWonder
#1
May9-12, 08:10 AM
P: 22
My science teacher says that a sphere will spin at a greater speed at it's poles. He says that the equator would spin the slowest.

He says it's basic physics that speed=distance/time.

Poles have a shorter distance to make a revolution contrary to the equator therefore it would have a greater speed.

The exact Axis point will not spin at all.

I tell him that as the circumference of a circle increases, a single point along it has to travel faster to complete a revolution in the same amount of time.

Am I right or wrong?

Thanks Guys.
God bless
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ZapperZ
#2
May9-12, 08:22 AM
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All points on the sphere will have a constant angular velocity (which is the number of rotations per unit time), but the circumference (or equator) will have the fastest linear speed, since linear speed v is measured as r*w, where r is the distance from the axis of rotation, and w is the angular velocity that I mentioned earlier.

http://hyperphysics.phy-astr.gsu.edu/hbase/rotq.html

So yes, you are correct.

Zz.
Whovian
#3
May9-12, 08:25 AM
P: 642
Did your science teacher really say that? Complain to someone who can fire them or move to a different class/school. :)

Quote Quote by Teacher
He says it's basic physics that speed=distance/time.
Yes. In fact, this is the definition of average velocity. And it does have less distance to go in the same amount of time. Therefore, its speed is lower, not higher. [itex]\vec{v}\ \alpha\ \Delta\vec{p}[/itex], so I don't see where your teacher is coming from.

Probably from a severe mixup of stuff. If a sphere has a certain Angular Momentum, and its radius suddenly decreases somehow, but Angular Momentum is conserved, its angular velocity will increase. So he might be thinking that points closer to the axis of rotation move more quickly?

KidWonder
#4
May9-12, 09:06 AM
P: 22
Sphere spins faster at poles/axis?

Thanks Guys, I knew I was correct.

And Whovian, my science teacher actually said that it was basic physics that Velocity =distance/time .

I'll have too explain the reasoning to him, it feels really good to prove my teacher wrong.

Thanks again.
Dickfore
#5
May9-12, 09:28 AM
P: 3,014
Is the sphere you're talking about rigid? Because, if it was a ball of gas, then, viscosity would cause points near the Equator to slow down, and points near the poles to have a bigger flow velocity, as compared to a rigid sphere. Thus, the angular speed of points near the equator would be lower than near the poles.
Gordianus
#6
May9-12, 10:56 AM
P: 217
I think the OP refers to a simple, rigid sphere. As for the teacher's knlowledge......
cjameshuff
#7
May9-12, 11:58 AM
P: 210
Quote Quote by Dickfore View Post
Is the sphere you're talking about rigid? Because, if it was a ball of gas, then, viscosity would cause points near the Equator to slow down, and points near the poles to have a bigger flow velocity, as compared to a rigid sphere. Thus, the angular speed of points near the equator would be lower than near the poles.
Viscosity in the absence of some disturbance would lead to a fluid sphere at equilibrium rotating as a whole exactly like a solid sphere. Cyclonic weather patterns and such occur because the atmosphere is being pushed around by things like solar heating, which still might confuse him. Or perhaps he was somehow getting confused by the speeds of orbiting objects?

In any case, the teacher's position is unsalvageable, his own reasoning leads to a conclusion opposite of what he claims.
Dickfore
#8
May9-12, 12:03 PM
P: 3,014
Quote Quote by cjameshuff View Post
Viscosity in the absence of some disturbance would lead to a fluid sphere at equilibrium rotating as a whole exactly like a solid sphere.
This is true.

Quote Quote by cjameshuff View Post
Cyclonic weather patterns and such occur because the atmosphere is being pushed around by things like solar heating, which still might confuse him.
Yes, or a nuclear reaction in the interior of the "ball"

Quote Quote by cjameshuff View Post
Or perhaps he was somehow getting confused by the speeds of orbiting objects?

In any case, the teacher's position is unsalvageable, his own reasoning leads to a conclusion opposite of what he claims.
The point is that he did not specify what the "sphere" was. When one uses terms like poles, and Equator, isn't it reminiscent of celestial bodies? Maybe the teacher was talking about the rotation of some celestial body, and the op pulled it out of context.
Borek
#9
May9-12, 12:36 PM
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Sun rotates with different speed at the poles than at the equator, but the difference is in exactly opposite direction - it is fastest at the equator.
Whovian
#10
May9-12, 12:38 PM
P: 642
Quote Quote by Dickfore
Maybe the teacher was talking about the rotation of some celestial body, and the op pulled it out of context.
Yea, something of the sort was one of my theories. Or perhaps the professor completely misspoke.
Dickfore
#11
May9-12, 12:42 PM
P: 3,014
Quote Quote by Borek View Post
Sun rotates with different speed at the poles than at the equator, but the difference is in exactly opposite direction - it is fastest at the equator.
True, and i believe the same holds for Jupiter as well.
Whovian
#12
May9-12, 12:47 PM
P: 642
And likely the other Gas Giants. (What is the Sun but a giant Gas Giant that's massive enough to initiate nuclear fusion in its core?)
Dickfore
#13
May9-12, 12:51 PM
P: 3,014
Well, here is the zonal wind speed distribution on Uranus:

http://en.wikipedia.org/wiki/Uranus#...nds_and_clouds

The direction of rotation changes as you go to the Equator, and the magnitdue is smaller than at the poles.
256bits
#14
May9-12, 08:04 PM
P: 1,409
Perhaps the teacher was talking about an object on a rotating sphere moving from the equator to the poles. I suspect the teacher has been misquoted.
KidWonder
#15
May9-12, 10:13 PM
P: 22
Quote Quote by 256bits View Post
I suspect the teacher has been misquoted.
I understand that you believe a High School Physics teacher would not say this but he did. I know exactly what he said, and we had a brief 3 minute conversation about the subject after school. His reasoning was because of the smaller revolution distance at the pole, it would have a greater velocity. I disagreed.

My teacher graduated with a degree in Geology but somehow teaches Physics.


And I was talking about a Rigid sphere.

I've recently came across this scientific theory that the reason the Sun magnetic shift every decade or so, had something to do with it's poles and equator spinning different velocities.

Pretty cool, eh?
willem2
#16
May10-12, 12:55 AM
P: 1,395
Quote Quote by Dickfore View Post
Well, here is the zonal wind speed distribution on Uranus:

http://en.wikipedia.org/wiki/Uranus#...nds_and_clouds

The direction of rotation changes as you go to the Equator, and the magnitdue is smaller than at the poles.
Note that the graph goes only from -60 to +60 degrees of latitude, and that the according to the article the maximum wind speeds are at -60 and +60 and the
speeds decrease to 0 at the poles.

The wind speed must decrease to 0 at the poles, because you'd a very large acceleration to make the wind go round a small circle round the poles. This is the same reason there's an eye in a hurricane.
Borek
#17
May10-12, 01:28 AM
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Quote Quote by willem2 View Post
The wind speed must decrease to 0 at the poles, because you'd a very large acceleration to make the wind go round a small circle round the poles. This is the same reason there's an eye in a hurricane.
I have read explanations based on the hairy ball theorem. There must be zero somewhere.
256bits
#18
May10-12, 12:25 PM
P: 1,409
I understand that you believe a High School Physics teacher would not say this but he did. I know exactly what he said, and we had a brief 3 minute conversation about the subject after school. His reasoning was because of the smaller revolution distance at the pole, it would have a greater velocity. I disagreed.
I stand corrected.
You are correct and your teacher needs to update his science knowledge.


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