The Moon is seen to keep one face toward Earth at all times. If viewed from a point directly above the plane of the planetary system, how does it have to rotate to maintain this alignment?

It must rotate once in approximately 27 days (once per month), or once per orbit around the Earth

Explanation:

The moon has only one of its sides facing the earth always. To keep up with this, it must travel round the earth in an orbit that takes approximately 27days

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.

e/c, where e is its total energy