Common mistake : Using equations without first specifying the convention used. Did you take up as positive or down as positive in the first instant.
The acceleration due to gravity is always down 9.81 m/s2 , if you have taken up to be positive , then a = -9.81m/s2 .
Common mistake 1: An object is dropped from a height 10 m above the ground . You have substituted s = 10 m without realising that you have assume down as positive and have used a = -9.81m/s2.
Common mistake 2: An object is thrown up at 2 m/s, find the time taken to reach the top? You have substituted u=2m/s without realising that you have assumed up as positive and have using a= 9.81 m/s2.
Common mistake 3: An object is thrown up at 2 m/s from a height of 2m and hit the floor, calculate its velocity. You have taken up to be positive but the ground is below the starting point and therefore s= -2m.
Once you have decided your initial velocity sign convention , the rest a, s, v must follow the same convention. Any slip would produce wrong answers to the whole question. The important thing is to let the initial velocity take the lead in deciding the direction up-positive or down-positive.
