Weight of a body is less in the mines. Why?
The weight of a body is less in the mines because when we move deeper the earth gravity decreases but our mass is always constant.
Why Newton's law of gravitation is a universal law?
Newton's law of gravitation states that the force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. It is the law that is valid and practical in the whole universe.
So, Newton's law of gravitation is a universal law.