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Velocity drop over 20m with a tennis ball (Read 684 times)
IronGoober
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Velocity drop over 20m with a tennis ball
Jan 16th, 2023 at 6:43pm
 
I just did this to try and figure out what is going on with my radar gun. Thought it was interesting and that I should share.

I put my camera at 120 fps about 35m from where I was throwing (to minimize parallax) and measured the speed of a tennis ball over the 20m. (used a range finder). From what I could tell, there was minimal parallax. My armspan varied by about 2.5% over the whole field of view and wasn't linear which was weird, may be from the swaying of the camera due to wind.

Not sure what else to say. It was a pretty new tennis ball with the "extra duty" fuzz on it.
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Re: Velocity drop over 20m with a tennis ball
Reply #1 - Jan 17th, 2023 at 4:55am
 
Smiley Good and interesting test  Smiley
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joe_meadmaker
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Re: Velocity drop over 20m with a tennis ball
Reply #2 - Jan 17th, 2023 at 7:12pm
 
The graph is a great visual representation.  Looks like a drop of around 10 m/s.

With the drop in speed happening that quickly, what is the thought on radar devices having a little variance in the reported speed based on when the object is actually detected/measured?
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IronGoober
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Re: Velocity drop over 20m with a tennis ball
Reply #3 - Jan 17th, 2023 at 11:33pm
 
I'm pretty sure their algorithms just report the highest "velocity" recorded.

Altthough I don't know all the guts, in essense, the unit pulses a radio signal out, it reflects off the traveling object and then it receives a radio signal back. It then takes a Fourier transform of that (giving frequency) and whatever the highest/lowest frequency it sees, it reports back the shift from the initial frequency as the speed (converted to velocity of course). If the object is traveling toward you  the frequency shifts up, if it's going away, the frequency shifts down.  So, as long as it can detect something far enough away, it should just report the highest velocity "seen".

I'm not sure how it determines what amplitudes of signals to filter out (like noise or other weak radio signals), but that can cause issues.

The other thing that could potentially cause issues is the end of the sling itself. The geometrical problem that if something is moving at an angle away from the radar gun, if it goes fast enough, the velocity away from the radar gun could be fast enough to be the fastest thing picked up. I thought I wasn't having this problem, but the above graph makes me unsure at this point. I'm going to have to do some more testing.

One thing that I can determine is that if i'm 1 or 2 m behind the radar gun and sling over the top, It affects the speed by 1 or 2 m/s (seems to be about 1m/s per m at the beginning).
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John R.
 
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IronGoober
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Re: Velocity drop over 20m with a tennis ball
Reply #4 - Jun 15th, 2023 at 2:50am
 
Here is a comparison of a "ready-man" glande (45g ellipsoid of polycarbonate) and a tennis ball's velocity drop over about 14m. This was done with a phone (240fps) and 5 m marked out for distance reference. The video analysis dropped a bunch of frames though. The measurement was pretty sensitive to position. A single pixel change in the measured position of the projectile could be up to 1.5 m/s difference in velocity. So it is pretty noisy.

I checked the FX chronograph against this data and it is pretty darn good. Especially for initial release velocity. If you sling over the top of it though, it takes a few meters for it to pick up the speed.

When throwing in front of the FX chronograph, a throw with a tennis ball and a glande both read 56 m/s. Over the top of the chronograph the tennis ball read 48 m/s, while the glande read 51 m/s. So there seems to be some distance in front of the radar gun where it picks up the projectile.
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Re: Velocity drop over 20m with a tennis ball
Reply #5 - Jun 15th, 2023 at 6:30am
 
This data set is very similar to what Ive experienced as well.
Tennis balls have very rapid decay in velocity.
And I get slightly higher readings in front of the radar compared to over the top .
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Re: Velocity drop over 20m with a tennis ball
Reply #6 - Jun 15th, 2023 at 8:44am
 
Mersa, the rapid drop in tennis ball velocity seems pretty logical -- light mass and large surface area/wind resistance are going to reduce velocity much more quickly than a similar mass but smaller size projectile.
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Re: Velocity drop over 20m with a tennis ball
Reply #7 - Jun 15th, 2023 at 10:50am
 
Nicely done IG!
So the ball loses roughly half its energy over 35m.  Now I want to know what the velocity dependencies are.  Assuming the air resistance loss is a function of velocity, is there a sweet spot? Or just an asymptotic relationship with diminishing returns?
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Re: Velocity drop over 20m with a tennis ball
Reply #8 - Jun 15th, 2023 at 8:03pm
 
Great visual comparison between the two projectiles IG! Thumbs Up
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Re: Velocity drop over 20m with a tennis ball
Reply #9 - Jun 16th, 2023 at 4:37am
 
Science!  Cool
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IronGoober
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Re: Velocity drop over 20m with a tennis ball
Reply #10 - Jun 16th, 2023 at 12:57pm
 
@NOOC Not sure. Apparently the terminal velocity of a tennis ball is around 20m/s (google). What sort of tests would determine what you are interested in? I'm not quite sure I understand what you are looking for.
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Re: Velocity drop over 20m with a tennis ball
Reply #11 - Jun 17th, 2023 at 3:43am
 
great work!

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