When you throw an object into the air, the object will decelerate by 10 or 9.8 meters per second per second due to gravity. When the object reaches its highest point, the objects velocity would be zero at a particular instant before it changes direction and falls back down. Why does the object change direction when its velocity is zero and why does the object reach its highest point when its velocity is zero? Please answer to this context.
It's based on Conservation of Energy, so the total of kinetic energy and potential energy must always stay constant.
Say you throw a rock upwards from the ground, (ignore your height for this part).
You provide the rock with kinetic energy, which is 1/2 mv $^2$, and it has initially no potential energy, it moves upwards against the gravitational field of the earth, until the velocity of the rock, and therefore it's kinetic energy is 0, but now it's potential energy is greatest, at a value of mgh, where h is the height it has reached.
Then, at v = 0, it has nowhere else to go but down, so it reverses direction and falls, losing potential energy but gaining kinetic energy, so preserving the conservation of energy law, until it hits the ground with the same kinetic energy it started with, and with no potential energy.
So, ignoring air resistance in both up and down directions, energy is conserved at all times.