use F = ma
F : force m : mass a : acceleration
so
f = 5kg * 20 m/s2 = 100 N
Faster than. Hope this helps!!!
Mass doesn't depend on where it is, and doesn't change.
The book's vertical position in the air is

where
. It reaches the ground when
, at a time
such that

So it takes the book 4 seconds to reach the bottom. The given initial velocity is irrelevant since it only has a horizontal component; vertically, the book is starting from rest.