How to "unjoin path" after joining path?

I made straight line, and then started an arc at one end
of the line to the other end of a line in order to draw
half circle.

Then selected both objects, right-click->Join path. OK. Now they
are joined as one object.

But how does one "unjoin" the path, to get back the original
2 separated objects before joining them?

I wanted to delete the straight line, but I can't since now it
is part of half circle as one object.

I selecting the half circle, then tried edit->ungroup, but this
had no effect.

Thank you
--Nasser