In the late 1500,Aristotle(great scientist of the era) gave up his theory that states objects fall at speed relative to their mass.At that time people take the words of Aristotle as the words written in THE BIBLE.

pisa

However Galileo, mathematician who work under Aristotle did not agreed to this. But he cannot argue without proof as it can cost him his job.

Later in a fine day, he went at the top of the leaning tower of Pisa, tried to draw conclusion while throwing two stones of different mass.
What he found was very much different that was proposed by Aristotle. He found that both the masses hit the ground at the same time.

 

Explaination from Physics point of View

An object that falls through a vacuum is subjected to only one external force, the gravitational force, expressed as the weight of the object.

W = m * g

An object that moves because of the action of gravity alone is said to be free falling.
The motion of any moving object is described by Newton's second law of motion, force F equals mass m times acceleration a:

F = m * a

We can do a little algebra and solve for the acceleration of the object in terms of the net external force and the mass of the object:
a = F / m
For a free falling object, the net external force is just the weight of the object:
F = W
Substituting into the second law equation gives:
a = W / m = (m * g) / m = g
The acceleration of the object equals the gravitational acceleration for any object.

The mass, size, and shape of the object are not a factor in describing the motion of the object.