Why that the slowing down of moving object is considered to be a form of acceleration?

Answer 1

Acceleration is defined by physics as the rate of change of velocity over time.

a = dv/dt

As time changes, any change in velocity results in a change in acceleration.

That change can be positive or negative.

If you want to know why, its because acceleration and velocity are vectors.

A vector has a magnitude and a direction.

The magnitude is the value, and the direction refers to the direction the object is traveling.

An example when acceleration doesn't point in the same direction as velocity is when you throw a ball into the air.

You throw it up, so the initial velocity is in the upward direction.

However the acceleration due to gravity is downward.

It will slowly decrease the upward velocity of the ball until it is zero.

At that point the ball will start to fall downard and increase in velocity until it hits the ground.