Question

why displacement is - ve, and zero
And why displacement =
final velocity- initial velocity /2​

Answer 1

Displacement is negative when the body is behind the initial point.

It is zero when initial and final positions are same.

S = v-u/2 is not entirely correct. I think this equation should be corrected

Answer 2

It is not neccesary that the displacement is always negative or zero. But it can be. This is because unlike distance it cares about the direction the object is moving. So, the displcement can either be positive, negative or zero.

Final velocity -Initial velocity /2 is the formula of average velocity not displacement.

The formula for displacement is Final velocity -Initial velocity because it is the change in position .

Hope u will understand !!