# Why do objects change its direction when their velocity reaches 0?

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.

• u dont need to make the velocity 0 to change the direction of the object. circular motion is one such example. the direction keeps changing in uniform circular motion. Commented Nov 20, 2015 at 9:08
• By default, when you throw an object up, the object changes direction when the velocity is 0.
– user97818
Commented Nov 20, 2015 at 9:16
• Doesn't "When the object reaches its highest point" sort of imply that the object does not go any further in that upwards direction? So it must change direction now (or stop) to avoid going even higher? Commented Nov 20, 2015 at 9:27
• @xander You said "When the object reaches its highest point, the objects velocity would be zero at a particular instant before it changes direction". it is partially true because only vertical component of velocity will be zero, not horizontal component, if the object is thrown at an angle (not equal to 90 degree). Commented Nov 20, 2015 at 9:52
• It's really a matter of frame of reference. There isn't a rest state for the ball it's just stopped moving relative to the observer. It is just convenient to call that 0 velocity. Commented Nov 20, 2015 at 11:46

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.