1. The problem statement, all variables and given/known data Example 3.13 : Example 3.1 2. Relevant equations v=u+at S=ut+1/2at2 v2-u2=2aS 3. The attempt at a solution I know the solution is in front of me, thats ok but I have a confusion regarding negative & positive signs. In both the examples something is moving vertically upwards & in both cases something is released below to the ground, then why in Example 3.13 initial velocity of the stone is taken as u = +12m/s whereas in Example 3.1 u = -2m/s ? Also in Example 3.13, displacement = -65 m, why ? Please clear these doubts of mine in as much detail as possible so that I know which sign to use in which case.