The reason that a 10 kg stone and a 1 kg stone, when dropped from the same height, reach the ground at the same time is due to the influence of gravity.
Gravity is the force that pulls objects towards each other. On Earth, gravity causes objects to accelerate downwards at a constant rate, known as the acceleration due to gravity. Near the surface of the Earth, this acceleration is approximately 9.8 meters per second squared (m/s^2).
When objects fall, they experience the force of gravity acting on them. According to Newton's second law of motion, the force on an object is equal to its mass multiplied by its acceleration (F = m * a). In this case, the force of gravity on the objects is proportional to their mass. The more massive an object is, the greater the force of gravity acting on it.
However, according to Newton's third law of motion, when the force of gravity acts on an object, the object also exerts an equal and opposite force on the Earth. This means that the Earth is pulled towards the objects with the same force as the objects are pulled towards the Earth.
As a result, the acceleration due to gravity is the same for both the 10 kg stone and the 1 kg stone. Since they experience the same acceleration, they fall at the same rate and reach the ground at the same time, neglecting any effects of air resistance.
This concept is often summarized in the famous story of Galileo dropping different objects from the Leaning Tower of Pisa to demonstrate that the mass of an object does not affect its rate of fall (disregarding air resistance).