Show that internal forces can change the kinetic energy but not linear momentum .

Explanation

`vecF_("ext") = M veca_(CM) implies vecF_("ext") = (M d vecv_(CM))/(dt)`
`vecF_("ext") = (d)/(dt) (M vecv_(CM))`

Only external forces can change the momentum of a system . Internal forces can gives rise to kinetic energy . A compressed spring attached to two blocks , converts its potential energy into kinetic energy . This K.E. is due to internal forces .