Would 100 joules break through a 2"x4"?

it sort of depends how it would hit the 2x4. It could bounce off a vertical 2x4 and compress it, or the 2x4 could be held very firmly by one end while the energy is applied to the opposite end - how it hits may make a big difference. It also depends how the energy is applied - maybe 100 joules of extremely concentrated laser energy might be enough to burn through the 2x4 and make it fall in 2, or a very fast-moving slug could make a hole in the 2x4 even if it is small. It seems to me like it might not be enough in any case! I think anyone will have a final answer til someone looks up some physical properties of a 2x4 ...if you want the answer to be 'yes' you could take a very cheap or very rotten 2x4 and then it might have the consistency of noodles. Then even your grandmother could break through it! ha!