Question

You need to pick up a book off the floor and place it on a table top that is 0.78 m above ground. You expend 1.56 J of energy to lift the book. The book has an acceleration of 1.54 m/s squared. What is the book's mass?

Answer 1

m g h = gain in potential energy
so
m (9.81)( .78) = 1.56
or
m = 1.56/(9.81*.78)

If it asked for the force, you could now do F = m a

Answer 2

The energy used to raise the book is m*g*h = 1.56J. I don't understand the rest. What weight?

Answer 3

It does not mention weight, only asked for m
There is no need to use a = 1.54 as the problem is stated.
I bet there is a part B though that asks for the average force
Then
F - m g = m (1.54)

Answer 4

oh ok yeah your right