A man weighs 90 N on the surface of earth of radius R at what height above the surface of earth is weight will be 40 N.

3/2 R

Explanation:

Let the Mass of man be m, Mass of earth be M.

By Newtons, law of gravitation,

F = GMm/R^2, Where G is gravitational constant

90 = GMm/R^2

When man weighs 40N,

GMm/(New R)^2 = 40

= 4/9 (90)

GMm/(New R)^2 = 4/9 (GMm/R^2)

(New R)^2 = 9/4 R^2

New R = 3/2 R

Therefore, height is 3/2 R

close lines strong magnet less close weaker

it doesn't. it was imagined up in the middle ages.