You need to make sure that the parent directory of new_module is on your python path. For a notebook that is one level below new_module, this code will do the trick:
import os
import sys
nb_dir = os.path.split(os.getcwd())[0]
if nb_dir not in sys.path:
sys.path.append(nb_dir)
If you're further down in the directory hierarchy, you will need to adjust the way nb_dir is set, but that's all. You should not run this code for a notebook in Jupyter notebooks, since it would add the parent of that directory to the python path, which is probably undesirable.
The reason the import works for notebook1 is that sys.path contains '' (the empty string), which is aways the current directory of the running interpreter (kernel, in this case). A google search for explain python path turns up several good explanations of how python uses PYTHONPATH (aka sys.path).
