chain reactions. the cure for the really really bored man. as we can see in this video the chain reactions are all set off by an initial push which gives the whole reaction enough energy to continue. the moving parts must have enough energy to activate the next portion, which must have enough energy to activate the next part and so on. the amount of energy changes due to most of the objects falling or rolling down hillls. momentum is not constant in all cases because friction has a lot to do with some of the reactions. the friction takes some momentum out of the reaction.