( ( when ( ( an object ) ( strikes ( ( another object ) ( in ( an ( inelastic collision ) ) ) ) ) ) ) ( ( -LRB- ( and ( ( ( ( yes ( , ( ( the collision ) ( ( ( is inelastic ) , ) ( unless ( you ( believe ( both walker ) ) ) ) ) ) ) ) and ) ( shell ( are ( going ( to ( ( BOUNCE ( off one ) ) ( another harmlessly ) ) ) ) ) ) ) -RRB- ) ) ) ( , ( momentum ( ( is ( ( conserved but ) ( ( not kinetic ) ( ( energy -LRB- ) ref ) ) ) ) . ) ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB when)) (S (NP (DT an) (NN object)) (VP (VBZ strikes) (NP (NP (DT another) (NN object)) (PP (IN in) (NP (DT an) (JJ inelastic) (NN collision))))))) (PRN (-LRB- -LRB-) (CC and) (S (S (INTJ (UH yes)) (, ,) (NP (DT the) (NN collision)) (VP (VBZ is) (ADJP (JJ inelastic)) (, ,) (SBAR (IN unless) (S (NP (PRP you)) (VP (VBP believe) (NP (DT both) (NN walker))))))) (CC and) (S (NP (NN shell)) (VP (VBP are) (VP (VBG going) (S (VP (TO to) (VP (VB BOUNCE) (ADVP (RB off) (NP (CD one))) (NP (DT another) (NN harmlessly))))))))) (-RRB- -RRB-)) (, ,) (NP (NN momentum)) (VP (VBZ is) (VP (VP (VBN conserved)) (CC but) (VP (ADVP (RB not) (ADJP (JJ kinetic))) (VBN energy) (-LRB- -LRB-) (NP (NN ref))))) (. .)))
when an object strikes another object in an inelastic collision (and yes, the collision is inelastic, unless you believe both walker and shell are going to BOUNCE off  one another harmlessly), momentum is conserved but not kinetic energy (ref.
(ROOT (S (NP (DT A) (ADJP (RB perfectly) (JJ inelastic)) (NN collision)) (VP (VBZ reduces) (NP (JJ internal) (JJ kinetic) (NN energy)) (PP (TO to) (NP (NP (DT the) (NN minimum)) (SBAR (S (NP (PRP it)) (VP (MD can) (VP (VB have) (NP (NN while)) (ADVP (RB still)) (S (VP (VBG conserving) (NP (NN momentum))))))))))) (. .)))