B Calculating Football RPM: Accuracy Guidance

[BonBonBrady]

looking for some guidance on what seems to be an oversimplification of calculating RPM. Using hi speed video (240 frames per second), I can determine that it takes .087 of a second for a thrown football to make its first revolution when coming off the hand of the thrower. Assuming I want to use this time for 1 minute, I get an RPM of 689.65. Does this sound correct? TIA

 

[DrClaude]

looking for some guidance on what seems to be an oversimplification of calculating RPM. Using hi speed video (240 frames per second), I can determine that it takes .087 of a second for a thrown football to make its first revolution when coming off the hand of the thrower. Assuming I want to use this time for 1 minute, I get an RPM of 689.65. Does this sound correct? TIA

Yes. If you divide 60 s by the time in seconds it takes to make une revolution, you end up with RPM.

 

[Mark44]

Alternatively, if you work with the units involved, it might be easier to follow.
Given information: $.087 \frac{\text{sec}}{\text{revolution}}$
This is equivalent to $\frac 1 {.087} \frac{\text{revolution}}{\text{sec}} = \frac 1 {.087} \frac{\text{revolution}}{\text{sec}} \cdot 60 \frac {\text{sec}}{\text{min}}$
The last factor converts seconds to minutes, and the resulting units are revolutions/min = RPM, with the seconds units cancelling.
Multiplying 60 by 1/.087 is equivalent to dividing 60 by .087, which is what @DrClaude suggested.

 

[Vanadium 50]

I get an RPM of 689.65.

Probably this is 690. When you wrote 689.65 you are saying it's more than 689.645 and less than 689.655. You probably don't know it this accurately.

 

[jedishrfu]

This article from Harvard says the average is 600rpm which correlates well with your measured result.

https://chandra.harvard.edu/blog/node/185

 

[Office_Shredder]

I would have thought a professional quarterback would get a lot more spin than a random person. Maybe it slows down pretty quickly after release?

 

[berkeman]

I would have thought a professional quarterback would get a lot more spin than a random person. Maybe it slows down pretty quickly after release?

I don't think so on either point. Even an amateur can throw a football 50 yards/meters, so the force of the throw and the corresponding release and release RPM would be similar. The difference with good quarterbacks in my experience is the much better alignment of the spin with the launch angle (no wobble) and much better accuracy of the launch path.

However, there is a very big difference in baseball backspin RPM between the casual baseball player and a trained high school / college / professional player, since we are taught to maximize backspin on the ball on our throws. We learn to always pull the ball out of our glove with our fingers in the "4-seam fastball" position, so we can whip the back of the ball down as we throw to maximize backspin and minimize the drop of the ball. I learned that in high school from our semi-pro baseball coach, and spent hours at home in front of the TV tossing a baseball into my glove and learning to always grab it in the 4-seam position. Thanks coach!