Just as Prof. Newton said: "For every action, there is an EQUAL, OPPOSITE reaction.". (See his motion law #3.)
If our person APPLIED a force of 80N pointing to the right, then that's the 'action'. It must follow, as the night follows the day, that the strength of the equal REaction is also 80N, and it points to the left because it's opposite to the action.
Answer:
Depends on how long the string is, how heavy the weight, and how high you let go of it.
But it will most likely hit you :)
the new momentum would be 40000 units as momentum = mass × velocity