In order for object to be accelerating, the net force (sum of all forces) acting on the object has to be non-zero. According to Newton’s Second Law of Motion, Force = Mass x Acceleration. Since the acceleration of gravity is always constant at 9.81 m/s2, the force of gravity is constant as well, according to Newton. If the object is sitting motionless on the ground, then the net force (sum of all forces) acting on it is zero. The force of gravity attracts object to the ground, but the ground pushes back with the force that exactly balances objects weight (force of gravity on the object). That is why the net force is zero and the object is motionless. If the ground didn’t push back, the object would simply fall through the Earth toward its center. The ground pushes back due to the electric repulsion between electronic clouds of the atoms in the ground and in the object.
Department of Physics
Adam Smith from New Brockton