The video is made with some text boxes that only appear on one frame. Stop on those frames to read them. This video pauses for 4 seconds on each frame with the brief text boxes. Normally, to move one frame on Youtube use the "." and "," keys. To get by the 4 second pauses use the video slider and "ms" time scale. (4 sec = 120 clicks of the "." etc.).
Closed Racket Angle before Impact
The closed racket angle should be videoed so that the racket is seen edge on to the camera just before impact. The measurement would be more accurate.
I put a 90 angle in the video, up from the court line, to act as a plumb line near the racket. See frame at 4 ms.
The racket angle at roughly 16 d. looks more closed than the 9 degrees that we measured in earlier serves. That closed racket face angle is one thing we were after for the bounce to the right.
But in the earlier serve comparing to Stosur your son was duplicating Stosur's kick serve technique very closely. I can't tell if anything changed when the camera angles change.
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Measuring ball speed.
Measure the ball diameter on your computer screen
before it is impacted - no ball distortions from impact. This will give a calibration of how many millimeters on your computer screen equals the ball diameter, ~ 2.6", in real space. You then can use this calibration and how far the ball moves in a known time to measure ball speed. However, the server was very wide on the baseline and served to an unknown spot in the ad court. If the camera were more aligned perpendicular to the ball's trajectory, the measurement will be more accurate.
From the image at
Frame at 0 ms - I measured the ball as 6 mm on my computer screen.
C - calibration factor
6 mm X C = 2.6" ball diameter
C = 2.6"/ 6mm
C= 0.433 "/mm The screen is calibrated near the impact.
Frame at 12 ms - ball position at 12 ms, Measure from center of ball.
Frame at 33 ms - ball position at 33 ms. Measure to the center of the ball.
I measure on my computer screen that the ball moved 48 mm.
D = 48 mm X 0.433 "/mm = 20.8"
The time, t, was 33-12 ms = 21 ms or 0.021 sec.
Velocity = D
/ t
V = 20.8"
/ 0.021sec
V = 990 "/sec (A useful conversion to remember is that 100 MPH = 1760" per second.)
V = 56 MPH, the component of forward ball velocity as seen by the camera. This 56 MPH speed would increase if the camera had viewed perpendicular to the ball's trajectory.
(Convert "/sec to MPH by entering "
990 inches per second to MPH" in the Google search box. (The Google search box works like a calculator).)
By placing a video camera squarely on the side of a ball's trajectory, its velocity can be measured.
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Correction to Measure Ball Speed
The server is serving from well to the ad sideline. I drew a line from the serve location to the right side of the opposite ad service box. Guessed the camera location and drew a line to estimate the ball trajectory. I measured some lines to get a correction.
The ball must have moved farther than seen across the camera's frame because it was also going away from the camera as it moved forward in the court.
The correction to speed is roughly 15/13 = 1.15.
V = 1.15 X 56 MPH
V =
64 MPH
In the Rod Cross article on the kick serve, he said that the high level kick serves are in the range of 80-100 MPH. See "Physics of the Kick Serve". Later I believe that Cross said in an interview that the 100 MPH used might have been on the high side. ?
