A0.64 kg bullfrog is sitting at rest on a level log. how large is the normal force of the log on the bullfrog?

... If nothing is moving, then nothing is accelerating.

... If nothing is accelerating, then the net force on the frog is zero,
and we know the forces on the frog's posterior are balanced.

... So the normal force of the log acting upward on the frog is
identical to the weight of the frog acting downward on the log.

... The frog's weight is (mass) x (gravity)

                                 =  (0.64 kg) x (9.8 m/s²)

                                 =    6.27 kg-m/s²

                                 =     6.27 Newtons

Awoodchuck would chuck all the wood he could chuck if a woodchuck could chuck wood.