When do i use these kinematic formulas (constant-acceleration)

[XwakeriderX]

Homework Statement

When do i use:

(delta)Y=V_iy+(1/2)a_yt²

Compared to...

(delta)Y=V_iy-(1/2)a_yt²

What tells me if I am going to be using
(+) or (-)

 

[PhanthomJay]

Homework Statement

When do i use:

(delta)Y=V_iyt[/color]+(1/2)a_yt²

Compared to...

(delta)Y=V_iyt[/color]-(1/2)a_yt²

Hi there.
Note my corrections to your formulae in red[/color]. Delta y must have length units. Watch your units!

What tells me if I am going to be using
(+) or (-)

Choose a direction as positive, say, positive is upwards. Then negative is downwards. So use plus signs for upward displacement, velocity, and acceleration, and use minus signs for downward displacement, velocity, and acceleration.

 

[XwakeriderX]

awesome thank you!

 

[snshusat161]

Sign Conventions are just a reference. They are not a must condition. Take whatever you like but be sure to use it every time and every where. You should never define sign conventions in two different way in the same equation and to save yourself from confusion you should never change the sign conversion in the whole problem.