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Re: The sign of g



Robert Cohen asks if g can be a vector pointing downward. If we do this, what does it represent? As others point out, g itself is not the acceleration.

In my mind, g is just a number. It is a shorthand way to write 9.81 or 32.2 or 981. Indeed, one nice thing is it doesn't have to take on a specific number until we decide what units we are using.

For a glider on a slanted air track the acceleration is gsin(theta)in the direction of the slant... and at this point we haven't had to specify if we are working in meters or centimeters or feet.

That's the reason I use g... (1) it is shorthand (2) it identifies the conceptual physical quantity that is needed, but it doesn't pin down the actual numerical value until we decide what units we are in or what planet we are on.

In this sense, I disagree with Rick Tarara when he says he prefers that g not appear in kinematics equations. It think it is significant and educational to identify this physical quantity when appropriate. The slanted air track is one example. Another example is the trajectory of a projectile. It think it is more than appropriate to put g in the trajectory equation rather than a. It tells us more information. It is not any old magnitude of a that goes in there; it is the g value for that particular location on earth or the g value for that particular planet. It helps us see how gravity is involved in the equation.

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817