The ratio of {HEAD ~ the outbound velocity} to {TAIL ~ the inbound velocity} is commonly referred to as the coefficient of restitution  
to
The ratio of { HEAD ~ the outbound velocity } to { TAIL ~ the inbound velocity } is commonly referred to as the coefficient of restitution
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH