力學問題問問大家~~

Elastic : momentum conserved , kinectic energy conserved
Inelastic : momentum conserved , kinectic energy not conserved
Completely inelastic is a just a case which two object stick together afterward

Say plasticine A and B of mass 2kg

Case 1 : uA = 10 , uB = 0 , vA = 0 , vB = 10
momentum conserved (before = after = 20)
kinectic energy conserved (before = after = 100)
air molecule always collide in this way , that's why they keep bumping into each other , but the energy (temp) wont drop

Case 2 : uA = 10 , uB = 0 , vA = 4 , vB = 6
momentum conserved (before = after = 20)
kinectic energy not conserved (before = 100 , after =52 )
energy is lost , but A and B dun stick together , B go faster

Case 3 : uA = 10 , uB = 0 , vA = vB = 5
momentum conserved (before = after = 20)
kinectic energy conserved (before = 100 , after =50 )
A and B stick together thus travel in same velocity
this is so called completely inelastic collision

this website should explain well enough for your question
http://www.sparknotes.com/testprep/books/sat2/physics/chapter9section4.rhtml

首先，momentum裡面並沒有分elastic定inelastic，是collision有分elastic or inelastic.所謂的elastic collision 就是沒kinetic energy loss 的collision，而 inelastic collision就是有kinetic energy loss 的 collision. 至於 completely or not 就要視乎kinetic energy loss 有幾多，全部loss咪叫completely.