Question

Why is the difference between the ground reaction force and the body weight equivalent to the net force acting on the person?

Answer 1

Normal force is the body weight equivalent force acting in the opposite direction of the body weight force(mg).It is perpendicular to the surface on which the object is residing.it keeps the body from falling through the surface.

Ground reaction force is the force exerted by the body on earth or ground.If a book is laying in a tabe then ground reaction force is the force exerted by book on ground not on the surface(table).This may be equal to normal force in some situations.But if the object is laying in an inclined plane then reaction force and normal force are different.Reaction force is explained by Newtons third law.