# When does acceleration due to gravity equal positive/negative? [closed]

For example a projectile is launched at an angle. What would $a$ in $y=vt +.5at^2$ be?

Let's say I choose up to be positive. How do you not confuse yourself whether to use positive or negative $a$?

## closed as off-topic by Danu, JamalS, Neuneck, ACuriousMind♦, JimOct 15 '14 at 12:37

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• See Floris's answer here. – HDE 226868 Oct 15 '14 at 1:42
• @HDE226868 would you say this question is a duplicate? – David Z Oct 15 '14 at 4:15
If up is positive, and gravity points down, then $a$ (acceleration due to gravity) would be downwards, so it will have a negative magnitude.