Question

a book accidentally falls from a shelf 4.2 m high.a librarian is standing nearby and moves .80m,starting from rest, to catch the book.what must be his average acceleration if he catches the book when it is 1.8m above the floor?

Answer 1

ok, when is the book at the ending position?
hf=hi-1/2 g t^2
t=sqrt(2.4*5/9.8) sec

Now, acceration.
d=1/2 a t^2
a=2d/t^2=1.6/t^2

Answer 2

320 metres per second