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 = z[sub]2[/sub] - z[sub]1[/sub]). 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/s[sup]2[/sup]) and velocity going down is negative.

Rinka

Ah! Thank you so much Dr_Ephemeron
!