W=m*g (1) where W=weight of an object , m=mass and g=acceleration of gravity.

The motion of this object as described by Newtons 2nd law is caused by a force F that is

F=m*a (2)

In case ONLY of free falling (this means that no other F occurs, not even resistance of air) the force that causes the movement is only the weight so

F=W

so (1) because of (2) becomes

m*a=m*g

Dividing both by m gets

a=g

What this means is that the acceleration of an object during free falling (a) is ONLY affected by g. No mass in the formula.

In other words, if you throw a nail and a tank from the 15th floor of a building and there is no air or any other force except gravity, they will fall down the same time with the same speed. Strange e? And because maths proves it, it is also true.