In the absence of air resistance, both the 1kg stone and the 20kg stone would fall at the same rate. This phenomenon is often referred to as the "equivalence principle" and is a fundamental principle of classical physics.
According to Isaac Newton's law of universal gravitation, the force of gravity acting on an object is directly proportional to its mass. At the same time, Newton's second law of motion states that the acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass. When these two principles are combined, the mass cancels out, and the acceleration due to gravity becomes independent of mass.
In simpler terms, regardless of their mass, objects near the surface of the Earth experience the same gravitational acceleration. This acceleration is approximately 9.8 meters per second squared (m/s²), often denoted as "g". Therefore, both the 1kg stone and the 20kg stone would fall with the same acceleration of 9.8 m/s².
It's worth mentioning that in real-world scenarios, air resistance does come into play, and objects with different shapes and sizes may experience different levels of air resistance, affecting their fall. However, in the absence of air resistance, as assumed in the question, the masses would fall at the same rate due to the influence of gravity alone.