Question

A rock is thrown vertically upwards. During its journey to the top, when is it that both its velocity and its acceleration are equal to zero?

Answer 1

At the highest point the velocity will be zero and acceleration due to gravity always present to the rock i.e g=9.8m/s2. directed in downward direction. It is possible only when we ignore the air drag force on the rock.

But if it is assumed to be existence of air frictional force acts upwards direction i,e opposite to the gravitational force causes decelerate the object. So its acceleration also going to be zero at a particular time where the relative density of air and object equal to 1. It can be simply expressed mathematically as follows,

Acceleration (a)= g(1-rel) =g{ 1-(_air/_rock)}

if _air = _rock hence a=0.