The velocity may be measured by the use of light screens, which measure the time required for {HEAD ~ the ball} to travel {TAIL ~ a fixed distance} 
to travel
The velocity may be measured by the use of light screens , which measure the time required for { HEAD ~ the ball } to travel { TAIL ~ a fixed distance }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL