I'm not sure what people mean when they say "the sign of g." Do they mean that g might be plus or minus 9.81, or do they mean there might be a plus or minus in front of g.
I agree with Joe Heafner and Bob Sciamanda that the number g does not have a sign... it is a magnitude... g = 9.8 m/s^2 or g = 32 ft/s^2... always.
It is often correct to put a negative sign in front of g, but it is also sometimes correct to put a positive sign in front of g. It depends on the orientation of the coordinate system.
If the positive y axis is up, and gravity is the only force, then accel = 0i - gj + 0k
If the positive z axis is up, and gravity is the only force, then accel = 0i + 0j - gk
If the positive y axis is down, and gravity is the only force, then accel = 0i + gj + 0k
If the positve x-axis points parallel to a ramp that is slanted downward to the right with ramp-angle theta, and the positive y-axis is normal to and upward from the ramp, and gravity and the normal force are the only forces, then... accel = gsin(theta)i + 0j + 0k.
It is especially is cases like this last one where students can get goofed up if they associate g with -9.8 rather than just the 9.8 magnitude.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
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