Definition of Physics Ch # 3

Motion under gravity: 

 If A body is thrown vertically upward with a certain velocity then due to gravity it's velocity decreases at the rate of 9.8 m/s 2,
      It means that the velocity of the body will decrease by 9.8 m/s in every second. As a result of which it's velocity becomes zero after it has-attained a certain height. At this point the body can not remain suspended in the air, therefore it starts falling down under the action of gravity with the same acceleration of 9.8 m/s 2-. This time its velocity .Increases by 9.8 m/s in every second. It hits the ground with same velocity with which it was thrown.
    During the entire motion under gravity, acceleration of the body is in the downward direction (towards the center of the earth) therefore it is taken as "-g" for upward motion, whereas for the downward motion it is taken as "+g ". The body takes a certain time "t " to reach the highest point. In the absence of air resistance it takes exactly the same time to return to the ground. Hence the total time for which the body remained in the air is "2 t "

Important note: 

For complete up and down motion g is to be taken as negative . The reason for doing so in such cases is that the direction of initial velocity vi of the body is taken as reference. It is taken as positive and any vector quantity including g , whose direction is opposite to the direction of vi will be negative. Especially in case of projectile motion g is taken as negative throughout the motion. Because at the point of projection the initial velocity in the vertical direction is upward whereas the direction of g throughout the projectile motion is downward. Hence g is taken negative at all the points, whether the body moves upward or downward. All the equations of projectile motion are derived on this assumption.