Question

gravitational force acts on all objects in proportion to their masses.why then ,a heavy object does not fall faster than a light object?

Answer 1

according to the second low of motion
F =ma
ie, mass and accelaration are indirectly proportional 
there for , when mass increases accelaration decreases . So the object is too heavy compared to other ,there is negligible or no accelaration at all 
so heavy object does not fall faster than a  light object

Answer 2

We know that :Force =mass *acceleration. Now, it is given to us that gravitational force acts on all object in proportion to their masses. That is possible only if the acceleration due to gravity is constant for heavy objects as well as light objects. Since the acceleration is constant , therefore, all the objects fall at the same speed