Question

This Discussion question, like the last one, deals with one-dimensional kinematics. A friend of yours contends that the acceleration of a ball tossed up into the air is positive on the way up, negative as it is falling down and zero at the top of the trajectory. What is your opinion on the matter? Justify your response based on kinematic principles.

Answer 1

Acceleration due to gravity is constant throughout the motion and its value always remains 9.8 m/s²

When the ball is tossed up, it start with some initial velocity lets take is positive as velocity is directed upwards and acceleration due to gravity is directed downwards so

so here acceleration is negative as it acts against the velocity vector and to which velocity decreases with time and finally drops to zero.

When balls reaches at maximum height, its velocity becomes zero but still acceleration acts downwards i.e. negative, because if there is no acceleration at the moment, ball must remains there it never comes down.

When ball start falling back, its velocity as well acceleration both are negative. So

So throughout the motion, acceleration due to gravity remains same. (-g) and negative sign ​indicates that acceleration acts downwards i.e. towards the center of the Earth.