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Old Mar 9th 2017, 06:01 PM   #1
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Can some body explain how the spin on the ball is accelarated

Hi ,
I am not very good at physics , But it would be appriciated if some body explain how and why the spin (revs) on the ball is getting faster (accelarated) between the ball leaves the hand and pitched to the ground
in the video
from 1:18 to 1:30

from 1:18 to 1:30
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Old Mar 9th 2017, 06:50 PM   #2
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Originally Posted by mailtobilATgmail View Post
Hi ,
I am not very good at physics , But it would be appriciated if some body explain how and why the spin (revs) on the ball is getting faster (accelarated) between the ball leaves the hand and pitched to the ground
in the video
from 1:18 to 1:30

from 1:18 to 1:30
Watching the slo-mo it doesn't look to me like the spin is accelerating. (There shouldn't be one...No torque on the sphere means no angular acceleration.) I do note that there is a "wobble" that it picks up in mid-air. This is due to the ball not being a perfect sphere and not having a center of mass at the center of the ball.

-Dan
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Old Mar 9th 2017, 08:14 PM   #3
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Hi Dan,
Thank Alot for reply infact I was not expecting a that quick reply, yes thats why i was surprised how without a tourque the ball is spining more i thought probably air is doing something or what but look at the shine brand mark on the left side of the ball, I think it start moving quickly when ball goes downwards in the loop , But I will check the exact time of each revolution in a slow motion timer app and then will come back to you ,
But many many Thanks for your response.
Bil
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Old Mar 10th 2017, 01:28 AM   #4
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I suspect this may be a strobe effect.
(Like the wagon wheels rotating the wrong way in old westerns).
I suggest that the actual spin rate is interacting with the camera frame rate to produce an illusion of a change in spin rate.
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Old Mar 10th 2017, 10:11 AM   #5
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Can some body explain how the spin on the ball is accelarated

The ball is starting to precess like a top. It does this because it is spinning and subjected to an external torque caused by the aerodynamic forces on the ball.

The rate of preccesion is inversely proportional to the rotational speed so by that logic, the rate of spin is actually reducing.

Maybe the precession makes it look like the rate of spin is increasing?
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