Question

A child throws a small ball vertically upwards. The ball is caught at the initial height 1.8 seconds after being thrown. What was the net displacement for the ball? What was the initial speed of the ball? What was the final speed of the ball? How high did it rise?

Answer 1

The net displacement was ZERO

That is a trick question
It ended up exactly where it started. It moved a distance, but did NOT displace.

Answer 2

Is the initial speed of the ball 0 m/s and the final speed 0 m/s?

Answer 3

t = .9 seconds up and .9 seconds down

now it is just like the other problem

0 = Vi - 9.81 t
0 = Vi - 9.81(.9)
Vi = 9.81*.9 which is about 9 m/s up

LOL, final speed down is the same as initial speed up also Vi

Note it asks for speed, not velocity, no plus and minus, just speed

h = 1.8 + Vi t - 4.9 t^2

Answer 4

So, can speed be negative, or is velocity only negative?

Answer 5

In your car you measure speed with your speedometer, no negative about it.
Tho get velocity you need both speedometer and compass, speed which is absolute value and also direction. If east is positive, west is negative.