Question

• What is point mass and unit mass?

Answer 1

point masses means ' whole mass of a big object is supposed to be concentrated at a point . 'If a big object has mass M , we suppose all mass of object is concentrated at a point , we don't take dimentions of object but consder it as a point . we do this to simplify derivations and equations.
for example - if a object is sphere and have mass m, to make equations we will suppose all mass acts at its center of gravity point and coside it as a point mass.

Actually there is no any concept as unit mass. Mass:

It is one of the fundamental quantity in physics, and the most basic property of matter, it is the measure of the amount of matter in a body. The SI unit of mass is Kilogram (kg)

Note: the mass of a body does not change at any time. Only for certain extreme cases when a huge amount of energy is given or taken from a body, example: in a nuclear reaction tiny amount of matter is converted into the huge amount of energy, this reduces the mass of the substance