Subject: What is the speed of a served tennis ball as it crosses the net?

This came up because we have a speed gun device that only measures ball 
speed as it crossed the net and we want to find out what the initial speed 
was when it left the tennis racquet. We know from a carefully done 
scientific study published on the web that a serve with an initial speed 
of 120 mph (193.2 kmph) has a typical speed of 87 mph (140 kmph) just 
before it lands near the opponent's service line. The serve travels about 
59 ft(18m) during that journey since it leaves the racquet about 1 ft 
inside the baseline. The distance to the net from the ball's point of 
contact with the racquet is 38 ft (11.58m). For purposes of the problem 
you can assume a rather flat trajectory serve. Given these parameters, is 
there a physics equation that will be able to tell us an approximate ball 
speed at any point in its flight given either its initial speed or given 
its speed as it crosses the net? Thanks.

Re: What is the speed of a served tennis ball as it crosses the net?