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Name: Tyler D.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: Thursday, November 28, 2002
Question:
Hey, 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?

Replies:
Tyler,

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 every object.

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
Physics Instructor
Illinois Central College


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