Discussion in 'General Pro Player Discussion' started by Schills, Mar 24, 2009.
Specifically, where do they aim the gun? At the net, at the server, at the service box?
The serve speed is measured as close as possible to the server. There was a recent post, I think by SystmicAnomaly, that showed how rapidly the speed of the ball went down even before it hit the ground.
If I remember correctly, a serve loses about half its speed by the time it gets to the returner.
I ask because I thought it might be fun to measure the speeds of the group of 25 guys I play with on the weekends. Seems like it would be tough to aim it in such a way as to ensure that the ball passes through the "beam". However, I have little knowledge of how these things work.
the radar gun is positioned along the net i believe, so it measures the speed of the ball as it passes the net which is why serves down the T is recorded much faster than serves out wide even if they are really the same absolute speed.
There must be at least 2 measurements at different points and the system must know the distance between those 2 points. The system then times how long it takes the ball to move between these 2 points. If you know how far something has travelled and you know how long it took, then you know how fast it was going as speed = distance/time.
with the radar guns you buy you could stand behind the server or at the net post as long as the aiming sot stayed constant and the serve location was constant you would be able to make a group comparison
just to clarify a few things about speed measurements using radar:
Q: where is it located?
- the radar is behind the server not at the net
Q: Where is it aimed?
- they don't have to be aimed that well. they have a somewhat large field of view. as long as the ball is in plain view of the radar it can measure the speed accurately (there are slight caveats to those statements but they are mostly true.).
- related to that, radars measure velocity along the line-of-sight from the center of the radar gun to the ball ... i.e. a straight line from the gun to the ball is the line along which the speed is measured. if the direction of the serve is hit off-angle from that "line-of-sight" which i just described, then indicated speed will be SLOWER than the actual serve speed. that is true for most serves, but the error induced is greatest for serves hit from the T out wide, but even more so for a doubles serve hit near the doubles alley.
Q: How is the speed calculated
- yes, speed can be measured as the time it take to travel between 2 points, but that is NOT that way radars work. RADAR guns measure something called doppler shift. They emit a radio signal at some frequency, and it get reflected back to the gun after "bouncing" off the ball. if the ball is moving the return frequency is different than the emitted freq. that difference is the doppler shift of the return signal, and is directly proportional to the speed of the object that is doing the reflecting. that freq shift is very easy to measure, and that is what a radar gun does. it does NOT measure a time difference. the doppler measurement happens very fast, and is effectively an instantaneous value of the speed. RADARS do not measured the time elapsed between to points.
Q: At what point is the speed measured?
- It's the initial speed, right off the racket.
- detailed answer: radar guns are setup to measure speed based a "first-return" signal, meaning they will measure the speed of the first strong signal it sees. there is some delay, but overall the measurement is made very quickly. it's fast enough that for all practical purposed, it measures the MAX speed of the serve as it comes off the racket. the racket itself contributes to the speed measurement, but since the speed of the racket is slower it will not be the indicated speed. the radar will return the highest speed of the strongest signal, which is the speed of the ball.
i was just about to correct you on that but nouser did it for me, yes any grade 10 student knows v=d/t but that isnt how radar guns work, have u ever used one? and i just remembered i used something similair in grade 12 cal, and its only logical for it to be pointed along the general path of the ball, so behind or infront of it. so i was wrong about the it parallel to the net
Radars pick up the ball right as it comes off the racquet. By the time it crosses the net it has lost half of its speed. Lol...If balls were PASSING THE NET clocked at 100 they would never be returned.
it´s a doppler radar, there fore it measures several points.
the doppler effect is aplied to any moving shape or element, sound, objects or light. it sends in signals that are reflected and recaptured measuring the distance... one signal one distance, another signal, another distance... several distances mesures within one time span lets the radar measure the speed
so in a nutshell, the radars measures at least two times... one first moment to pinpoint and a second one (several ones actually) to calculate the distance went, and therefore the speed at wich the object traveled.
i hope this graphic helps...:
this or whatever mr shaolin "i dont like shrieks" says... that is more scientifically acurate...
There is some kind of thread at TW that refers to "qualifying" members for a 100mph+ club or something. And there was an applicant who sent in a utube reference which showed how the radar thing functions. Don't remember much more about it but there were two people actually involved. One a white guy who was trying to join the club and one a black guy who served really hard. Sorry I can't be more helpful but, if you search, you should find it. Probably somewhere under Miscellaneous.
You're right, I've essentially described how many speed cameras work rather than radar based speed 'guns'.
I think Gorecki has nailed it now though.
WTF is YOUR problem? Yeah, I dont like shrieking/screaming from Larcher De Brito. About a hundred people said the same thing. Go get a girlfriend and dont single me out like that, wierdo. You are probably in the minority anyway if you like shrieking.
TENNIS magazine did an article on the way serve speeds are measured and the speed is measured from pretty much the moment the ball is hit, at its fastest. But thanks for enlightening us with you nice graphic and everything.
I don't know how it is done, but this is not the only way to do it. The way speeds are measured these days is by measuring the Doppler shift in frequency of the reflected wave to the incident wave.
hey.. no need to shout or shriek when someone points you out on your ignorance about 12th grade physics.
your comments are not correct.
doppler is doppler ... it is not distance, and it is not time.
doppler radars measure doppler shift which is a frequency. the frequency is the related back to velocity. doppler radars are incapable of measuring time and distance.
speed_of_ball = speed_of_light * doppler_shift / base_radar_frequency
Wow....sorry I spend my time playing tennis while you are researching physics, living in your mom's basement.
my comments are toned down to the audience. or do you want me to develop my perceptions on the aplication and usage of the effect derived from the shift in frequency and\or wavelength of waves which results from a source moving with respect to the medium, a receiver moving with respect to the medium, or even a moving medium as apllied to a machine able to interpret and convert it to speed of a miving object.
perceived frequency (f ´) wich is related to the actual frequency (f0) and the relative speeds of the source (vs), observer (vo), and the speed (v) of waves in the medium by
you got me all figured out havent you...?
your equation is correct, and it is the full, general, doppler frequency equation. mine was the actual speed estimate, based on a simplified doppler equation assuming a stationary source and use of radio waves ... which are what radars use.
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