jbriggs444 said:
My preference is otherwise. For me, ##g## is the named constant for the magnitude of the acceleration of gravity at the Earth's surface. Because it is a magnitude, it is always positive.
For me it's more than a preference, it's a MUST to avoid confusion when free body diagrams (FBD) and their translation into an equation through Newton's second law is involved.
Example
A 3 kg mass is hanging vertically from a string in which the tension is 50 N as shown in the figure on the right. Find the net force acting on the mass.
Solution
1. Draw the FBD as shown on the right. Gravity defines the "down" direction. Clearly the direction opposite to "down" is "up".
2. The net force is the vector sum of all the forces and you want to write an algebraic equation for it. Before you do that, you have to choose which direction is positive and which is negative then proceed with the solution.
Choice I
"Up" is positive
The net force is the sum of what is "up" with a "+" sign up front and what is "down" with a "-" sign up front. F
net = +T + (-mg) = T - mg = 50 N - 2 (kg) * 10 (m/s
2) = 50 N - 20 N = 30 N.
Since "up" is positive and the algebraic result is positive, the answer is
Answer: The net force is 30 N up.
Choice II
"Down" is positive
The net force is the sum of what is "up" with a "-" sign up front and what is "down" with a "+" sign up front. F
net = (-T) + (mg) = -T + mg = -50 N + 2 (kg) * 10 (m/s
2) = -50 N + 20 N = -30 N.
Since "down" is positive and the algebraic result is negative, the answer is
Answer: The net force is 30 N up.
It should be clear how either choice gives the same answer. It should also be clear that, when the time comes to replace g with a number, that number must be positive in either choice because g is the magnitude of a vector as noted by
@jbriggs444. The direction of the acceleration of gravity has already been taken care of by the transcription of "up" and "down" into "+" or "-" when one casts the FBD in algebraic form.