# Q & A: Falling stones

Q:
hi everyone. i have two stone of same density with different mass (i.e 1kg and 100 kg) if i push both the stone from a building of 100 ft. which one will reach ground fast. explain me if answer is, both will reach at same time. and what in case if i take two stone of different density with same mass..?
- venkat (age 12)
india
A:
Hello Venkat, nice question.
There are two main forces acting on a falling body:  the gravitational attraction and the effect of air resistance.  If only the force of gravity were acting then the two stones would arrive at the same time.   Galileo's thought experiment about the effect is described in:

The effect of air resistance is determined by the shape and surface area of the falling body.  Given that the two bodies have the same density and shape, then the heavier one will have a larger surface area and the retarding force will be greater.  At the same time the force of gravity will scale as the mass of the object.  The ratio of these two forces tends to favor the heavier stone falling faster since the gravitational force is proportional to the mass, which grows as R3 , whereas the air resistance force can only grow as R2.   R is the radius of the stone.

LeeH

(published on 12/10/2009)