Question

If energy is conserved, then:
A.PE(before) = KE(before).
B.initial (PE + KE) = final (PE + KE).
C.the momentum does not change.
D.the initial KE must be zero.

Answer 1

if you have a book up in the shelf, it has PE1=mgh and KE1=0.5*m*v^2=0 (doesn't have velocity).
If it drops, on the moment it hits the floor PE2=mgh=0 and KE2=0.5*m*v^2.
Where PE1+KE1=PE2+KE2.

Answer 2

b.

Answer 3

?

Answer 4

yes you are right :)