Question

When an object falls down why is acceleration of gravity not positive if both acceleration and velocity are both pointing in the same direction toward the earth and speeding up?

Answer 1

Great question! The answer is a matter of perspective.

You’re right, from the ball’s perspective, it is accelerating with a positive value.

However, from the earth’s perspective, the ball is approaching the earth, and therefore the ball’s acceleration is negative.

Answer 2

You can define z as positive up in which case both g and the resulting eventual velocity down are negative.
in this case:
a = -9.81 m/s^2
v = Vi - 9.81 t ,where Vi is initial speed UP
z = initial z + Vi t - 4.9 t^2 [ this is the usual way ]
NOW, if you define z as positive DOWN
a = +9.81
v = Vi + 9.81 t , where Vi is initial speed DOWN
z = initial z + Vi t + 4.9 t^2

Most of us call z positive UP and g down :)

Answer 3

www.bartleby.com/solution-answer/chapter-26-problem-66pq-physics-for-scientists-and-engineers-foundations-and-connections-1st-edition/9781133939146/a-500-nc-charged-particle-is-at-point-b-in-a-uniform-electric-field-with-a-magnitude-of-625-nc/9257f8b6-9734-11e9-8385-02ee952b546e