A ninja applies a force of 1250-N to a piece of 4-inch solid oak with his fist. What is the size of the force applied to the fist by the piece of oak?

The force is 1250-N

Explanation:

Generally according to Newtons third law , which states that for every action there is an equal and opposite reaction

 Now considering the question, the action here is the force applied by the ninja while the reaction is the force applied by the oak and from Newton law both of these action are equal but opposite to each other.

Hence the force applied by the the piece of oak on the fist is 1250 N