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 -?