Question

What is the reaction force for the force of gravity acting on an object?

Answer 1

The normal force is a force perpendicular to the surface on which the object is laying. It is what keeps the object from falling through the surface. The reaction force of gravitational force is the force an object exerts on EARTH.

Answer 2

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The opposing force is the gravitational force exerted by the person onto the earth equal in magnitude (Newton's Law of Gravity). This force produces an acceleration (Newton's Second Law) but because the mass of the earth is massive as compared to the human body, it is relatively small and can be taken as zero