Solving Newtonian Problems: Deciding When to Use + or - 9.8

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7tongc5
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ex) you throw a baseball straght up. I returns after 3.5 sec. How fast did you throw it?

i could then use x = x(initial) + v(initial)*3.5s + 1/2(-9.8)(3.5^2) and solve for initial velocity which comes out to be 17.5m/s.

in this problem, 9.8 was negative and in some other problems, 9.8 was positive. how do i decide when to set 9.8 + or -?
 

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It depends on how you define your coordinate system for the problem. It really does not make any difference as long as you are consistent.