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Hi.Sr. I would like to know when the gravity is positive or negative

Hi joeotilio25, thanks for your question. Gravity is a force, just like any other, and it's sign depends on how you've chosen to define your coordinate system. Traditionally, positive is chosen to be upwards, which makes any vector pointing downward, be it velocity, displacement, acceleration, or force, negative when pointing down. In that coordinate system, the force of gravity would be negative since it's pointing down. So, gravity is negative in a coordinate system where up is positive, and gravity is positive in a coordinate system where down is chosen to be positive.

Hope this helps,
Mr. Dychko

Hi Sr. I am wondering what happened to the other "Y" when you were solving for time. It's the part where t = square root of 2Y/AY.

Hello, thanks for the question. The "y" in the denominator is just a subscript for the "a" (acceleration), to label the acceleration as "the acceleration in the y direction". It is not a separate factor, but maybe I wrote it a bit too big. The equation is $t = \sqrt{\dfrac{2Y}{a_y}}$.

All the best,
Mr. Dychko