[Rinka]

Umm... (I must sound like an amateur)
In SUVAT equations...
How can you tell when a displacement has to be negative or positive..?

Like when freefall... is the displacement +ve or -ve
:-[
(if down is considered +ve)

And when you throw something up (considering up as positive)
is displacement +ve or -ve

So sorry!

Please help and Thanks

[Dr._Ephemeron]

You can make it anyway you want, but all vectors must be in the same reference frame.

If you make down positive, then displacment down is positive. This is counter-intuitive, but it works.

Just make up positive and down negative and always stick to using that! That way all displacements up are positive.

Also, remember that a change in something, such as position, is always the second position minus the first position (delta_z = z₂ - z₁). So if down is positive and you go from 200 m to 100 m, that is a displacement of  100 - 200 = -100 m wich is a negative value. This is wrong! Down is positive so you must be going from either 0 m to 100 m down or 200 m to 300 m down for it to be positive. So it makes more since to make up positive so going from 200 m  to 100 m is correct! This means g is negative (-9.81 m/s²) and velocity going down is negative.

[Rinka]

Ah! Thank you so much Dr_Ephemeron
!