Falling Bodies, Air Resistance, Mass
Name: Tyler D.
Date: Thursday, November 28, 2002
we are doing some research about falling bodies. My class and myself
would like to know what exactly is the relationship between a falling
object and the air resistance behind it. Air resistance will vary with
the mass of an object, but is there a way to measure it?
The "effect" of air resistance varies with mass, but the force itself does
not. What air resistance depends on is speed through the air: how many air
molecules must be pushed out of the way in a second's time. Shape and size
also affect this. An aerodynamic object allows more time to move the air
molecules aside. A small object doesn't need to move as many.
There is no exact formula for air resistance. For slow velocities (feather
falling, ball tossed lightly into the air), air resistance is proportional
to speed. For fast velocities (rain drop, airplane), air resistance is
proportional to the square of the speed. For many cases, air resistance is
somewhere in between. Exact formulas for air resistance are different for
To measure air resistance for a falling object, measure its acceleration in
a specific situation. One way is to use a ticker tape attached to the
object (and of course some sort of ticker). Calculate velocities from the
positions. Calculate accelerations from the velocities. Another is to use
a motion sensor attached to either a computer or calculator data acquisition
system. Vernier offers some good options. You cannot use the "constant
acceleration" formulas because the acceleration is not constant.
A falling object has two forces: gravity downward (mass*9.8m/s^2) and air
resistance upward. Together these combine to form the net force. Isaac
Newton tells us that this combination of forces must equal
mass*acceleration. From these forces, calculate the air resistance. Look
at how it changes as the velocity of the falling object changes.
Dr. Ken Mellendorf
Illinois Central College
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