Acceleration (the rate of change of velocity) is caused by a net force on an object. On Earth, one force we can always count on is the ever present force of gravity pulling down on any object that has mass. If gravity is the only force acting on an object, then we find the object will accelerate at a rate of 9.8m/s^{2} down toward the center of the Earth (this is often rounded to 10m/s^{2}).
In fact, all objects will fall at this rate if the air does not act to slow them down. An easy proof of this is to drop a grape and a grapefruit at the same time from the same height.  
However, if you drop a book and a piece of paper, you will notice
that the paper does not accelerate as much as the book because the small force of gravity
on the paper is easily opposed by the force of air friction on the paper. A similar
force acts on the book from the air but it is small compared to the force of gravity on
it. To prove to yourself that it is the air, put the paper on top of the book then
drop the book (try it).

©1999 Science Joy Wagon