{TAIL ~ The balls} are fired at {HEAD ~ a massive rigid block}, with the inbound and outbound velocities being measured 
{ TAIL ~ The balls } are fired at { HEAD ~ a massive rigid block } , with the inbound and outbound velocities being measured
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH