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03 October 2011

What does g mean?

(From a note to my class folder after a lab writeup:)
From NASA:  Recognize the guy in "zero g"?  (Or *is* he
in "zero g"?)

The variable g represents the gravitational field, which near earth is 10 N/kg.

Or, the variable g represents the free-fall acceleration, which on earth is 10 m/s2.

The variable g does NOT represent the "force of gravity" or the "gravitational pull."  The force of gravity on an object is the object's weight, or mg.

The variable g does NOT represent the "free-fall velocity."  Such a thing does not exist.

And finally, the variable g does not mean "gravity."  That's ambiguous -- lost of quantities are associated with this nebulous thing called gravity.  There's gravitational field and free-fall gravitational acceleration, but also gravitational force, gravitational potential energy, and the universal gravitation constant.



  1. Wow... Dr. Hawking looks like a kid at the carnival! Who ever said physics was fun?


  2. This distinction about what "g" stands for is something I always struggle to get students to be consistent about. They all just want to call it gravity, or even worse, FORCE of gravity! Despite continuous corrections and class discussions to the contrary. Any suggestions on how you get students to be consistent?

  3. Jessica, it’s a battle of attrition. I use the quiz in this post:

    I’m grading 2019 AP1#2 right now, and I’m seeing every possible misstatement about the meaning of g. I don’t think bullwhips are allowed in schools anymore, right?