Why do we cut a tree down, but chop it up into firewood?

With phrasal verbs sometimes it's merely collocation. IMO, this is one of those cases.

Chop down is an obvious use of a literal meaning of a preposition but I think chop up is just collocation. It doesn't seem to take either the literal meaning of "up" or any of the implications of "up" common to phrasal verbs.

Agreed with Craven.

It seems that most verbs that describe cutting something into a number of smaller pieces can use "up".

E.g. chop up, slice up, cut up, dice up, carve up, hack up, slash up, etc.

The colllocation seems to have too much rights to persuade our poor imagination to trust its authority.

...so it'll fit in the fireplace?

Good one, Jer.

Does a house burn down or burn up?

OW! ...I felt something in my brain pop.

It depends on what you wish to emphasize in your sentence -- the flames, or the fact that you're losing your house. The house is falling down as the flames are rising up -- I think that justifies the use of opposite terms here. Both are examples of using the literal meaning of a preposition.

Yeah, but you can go back and forth on the subject all day.

and how come you have to go back before you go forth????


And if your forth, who's on first?

Like Scoates, I I think I heard something in my brain pop. No, wait! It just exploded! Thanks Joe. You should have been a Zen master.