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