If the universe is expanding, why we're not expanding too? we're part of the universe too?

The universe expansion is defeated in small scale areas due to the small distances involved and the idea that gravitational force has less time to grow in small scale areas(where living creatures are located) unlike in large scale areas(the entire universe)

Explanation:

The meaning of 'the universe is expanding ' is that the galaxies outside the one we are in are moving away from us and those far away are moving the furthest.Galaxies are moving in space and because the universe has no center, everything is moving away from everything else.However, the universe is not uniform and perfect due to presence of over-dense areas like the planets and stars or the under-dense areas like the cosmic voids,small scale areas like those inhabited by living creatures are dominated by forces of electromagnetic and nuclear forces which other physical phenomena are at play instead of the 'universe expansion'.

the velocity you are looking for is made up of two parts: horizontally velocity, vh, and its vertical velocity, vv. the actual velocity of the ball will be these two parts added together, and because velocity is a vector then they will need to be added vectorially.  

neglecting air resistance etc etc.,  

the horizontal velocity component will be constant (because there is no accelerating force in the horizontal direction.  

vh = 8 ms-1  

the vertical velocity component has an accelerating force: 'g'.  

using v = u +at where a = g = 9.81 ms-2.  

the vertical component of velocity, vv = 0 + gt = vv =9.81 x 2  

vv = 19.62 ms-1 . the zero appears because the virtical component of velocity at time t=0 is zero. remember, the ball was thrown horizontally.  

how fast it will be moving (it actual path (being a parabola )) will be the vector sum of these two.  

adding vectorally gives: -  

v = sqr root (8^2 + 19.62^2) = sqr root(64+384.9) = 21.18 or 21.2 ms-1