Newton's third law can be summarized as "every action has an equal and opposite reaction". In this problem, consider the action of the entire Earth on a person standing at rest on the ground and the opposite reaction, where ‘action’ and ‘reaction’ are understood to mean ‘force’ and ‘reaction force’

What is the "equal and opposite reaction force to the gravitational force of the Earth acting on the person?

(1) The contact force of the ground on the person, keeping them from traveling towards the center of the Earth.

(2) Since the person is not moving, there is no reaction force in this situation.

(3) The mutual gravitational interaction between the Earth and the person.

(4) The gravitational force, exerted by the person on the Earth.

The gravitational force, exerted by the person on the Earth.

Explanation:

Using newton third law of motion which states that " for every action, their is always equal and opposite reaction "

For a man standing on the surface of the earth, there are two forces acting on the man, which are his weight acting downward and the normal force perpendicular to the plane.

Note the weight is due to gravitational pull on the man and it is given as, W=mg.

Now, applying

ΣF=ma

Since the body is on the earth surface and it is not moving downward, then, a=0

ΣF=0

N - W =0

N=W=mg

Then, this show that the action and opposite reaction is the gravitation force the earth exert on the person.

Then, the normal force is equal to the weight.

So, the best answer is number 4 which say "The gravitational force, exerted by the person on the Earth."

the doppler effect is used by weather forecasters to measure 

moving clouds