 Newton's Third Law

Action and Reaction Forces

Physical Science 112: Written by H. Tahsiri

An apple in free fall accelerates toward the Earth with a free fall acceleration, g. The force of the apple on the Earth also causes the Earth to accelerate toward the falling apple. By Newton's Third Law, the force of the Earth on the apple is exactly equal and opposite to the force of the apple on the Earth. By Newton,s Second law, the force of the Earth on the apple is equal to the mass of the apple times g , the accelerations due to gravity. And, the force of the the apple on the Earth is equal to the mass of the Earth times the acceleration of the Earth toward the apple. In conclusion, the magnitude of the forces are equal, or

F ( apple on the Earth) = F( the Earth on the apple) or

M( mass of the earth) x a( the acceleration of the earth toward the apple) = m(mass of the apple) x g( the acceleration of the apple toward the Earth) or

a = (m/M) g

where g= 9.8 m/s/s

This ratio of the masses is about 1/10^24. The Earth's acceleration due to the pull of the apple is so small, it is impossible to detect.