How to know whether it is +,- g and +,- y in the kinematic equations?

[bluesteels]

TL;DR

i was doing some problem and i have a hard time figuring out when will the y be positive and negative and same with the gravity idk if it -9.8 or 9.8

i was doing some problem and i have a hard time figuring out when will the y be positive and negative and same with the gravity idk if it -9.8 or 9.8

 

[anuttarasammyak]

If +y has same direction with gravity, it is 9.8.
If +y has opposite direction with gravity, it is -9.8.

 

[PeroK]

Summary:: i was doing some problem and i have a hard time figuring out when will the y be positive and negative and same with the gravity idk if it -9.8 or 9.8

i was doing some problem and i have a hard time figuring out when will the y be positive and negative and same with the gravity idk if it -9.8 or 9.8

There's no way to figure it out. It's your choice! Gravity is always directed towards the Earth - there is no choice about that.

You can also use $z$, rather than $y$, for the vertical coordinate, if you want.

 

[vanhees71]

It's a great idea to study vector algebra (and some calculus) in parallel to learning mechanics. Compared to the confusion some intro textbooks avoiding this utmost important mathematical tool for all of physics, it's a little effort, and it helps to avoid the typical confusion about signs discussed in this thread!

 

[Herman Trivilino]

If +y has same direction with gravity, it is 9.8.
If +y has opposite direction with gravity, it is -9.8.

Most textbooks would already insert the minus sign in the latter case, so you would still replace g with 9.8.

 

[Herman Trivilino]

Summary:: i was doing some problem and i have a hard time figuring out when will the y be positive and negative and same with the gravity idk if it -9.8 or 9.8

i was doing some problem and i have a hard time figuring out when will the y be positive and negative and same with the gravity idk if it -9.8 or 9.8

Every rextbook does it differently. You would need to look at the relevant chapters and read some of the worked examples.