When is the acceleration of a body is positive, negative and zero?

The acceleration is positive when it's in the same direction as that of velocity. It keeps on increasing the velocity. In this case, velocity is not constant.

The acceleration is negative when it is in the direction opposite to that of velocity. It keeps on decreasing the speed of body.

The acceleration is zero when the body moves with a constant velocity in any particular direction.

that tool is called an altimeter which measures your altitude which can be used to tell which atmosphere you are currently in, including the stratosphere.

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