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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 $a$?

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closed as off-topic by Danu, JamalS, Neuneck, ACuriousMind, Jim Oct 15 '14 at 12:37

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  • $\begingroup$ See Floris's answer here. $\endgroup$ – HDE 226868 Oct 15 '14 at 1:42
  • $\begingroup$ @HDE226868 would you say this question is a duplicate? $\endgroup$ – David Z Oct 15 '14 at 4:15
  • $\begingroup$ @DavidZ, yes, I'd say so. Although it asks about acceleration specifically, the question HDE linked asks about "all the y components", which includes acceleration $\endgroup$ – Pranav Hosangadi Oct 15 '14 at 5:41
  • $\begingroup$ @Pranav (and HDE) the implication of my last comment being please flag it as such if you think it's a duplicate ;-) $\endgroup$ – David Z Oct 15 '14 at 6:02
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It depends on what direction you assign to be positive in your coordinate system. To avoid confusion, just remember which direction acceleration is acting and which direction you assigned to be positive.

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Well, things don't fall up, so gravity would be negative. The longer version: For classical mechanics, the direction of a coordinate system is often arbitrary. For free fall problems I generally choose down to be positive (especially when I'm not dealing with an initial velocity, but it's a matter of preference). What you do need to know, no matter what though, is that once you've picked a direction to be positive, all other vectors acting along that axis are also positive. Vectors acting in the opposite direction car a negative sign.

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If up is positive, and gravity points down, then $a$ (acceleration due to gravity) would be downwards, so it will have a negative magnitude.

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