Question

Why this happens when 2 objects of different masses reaches at the same time to th ground ?

Answer 1

All objects, regardless of mass, will fall at the same rate.

The heavy and light objects travel at the same rate because
there are two competing factors that cancel each other out.

The force of gravity is greater on the heavier object than
on the lighter object, proportional to the object's mass.
This means that an object with twice the mass will be pulled toward
the earth with twice the force.

On the other hand, the acceleration is proportional to the force
divided by the mass. This means that an object that is
twice the mass of another object will be accelerated
twice as slowly as the lighter object given the same force.

So in order for an object with twice the mass to move
at the same rate as the lighter object, the heavier object
must be submitted to twice the force. And this is exactly
what the force of gravity does.