Answer to Question #8209 in Mechanics | Relativity for shahera
when you are standing on the ground how much upward force is exerted on you by the ground?why does this force not lift you up?
By Newton's Third Law: For a force there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.
Hence, since the downward force (your mass x gravitational acceleration) causes a reaction force that is equal in magnitude and directly opposite in direction, the net force will be zero. Hence, by the manipulation of Newton's second law: net Force = mass x acceleration.
If the net force = 0, and mass is still your mass, acceleration has to be 0. If there is no acceleration when you are at rest (meaning, no change in momentum) it means that you remain at rest. Hence, no force is produced to lift you upward.