Question

In which all cases should we take gravitational acceleration 'g' as negative and where should we take it as positive?

Answer 1

The sign of acceleration due to gravity (g) can be both negative and positive, it depends on the sign convention you wish to follow

If you choose downward direction to be positive and upward to be negative (generally used convention) then,

1. g will be positive if
3. the object is stationary or moving downwards
4. g will be negative if the object is moving upwards

If you choose the sign convention that is completely opposite i.e. upwards being positive and downwards being negative then the value of g changes accordingly

In short, the value of g depends upon the sign convention you wish to follow and the direction of motion of body under observation.