# Applicability of Newton's third law

Suppose you apply a force (F) on a wood block which is resting on a table. You increase the applied force from zero. We know that the static frictional force is acting on the block in the direction opposite to the applied force, F. However, this is not an action reaction pair. Does this situation obey newton's third law? My idea is that this situation obeys newton's second law not the third law. Should action reaction be always paired in order to say an action has an equal an opposite reaction?

• Newton's third law states that once you apply a force on the wood, the wood (and not any static reaction) applies a force on you too, which is exactly what happens. – gented Sep 13 '17 at 21:18

Newton's third law would apply to the force $\vec{F}$ which your hand (or finger or foot) exerts on the block in this way: there is an equal magnitude force $\vec{F}'$ which the block exerts on your hand.
The static friction force arises from the block's interaction with the table, and it is a force, $\vec{F_s}$ which the table exerts on the block. In your situation it happens to be opposite the direction of $\vec{F}$, but it is not the Newton's 3rd law partner. The 3rd law partner force is a force which the block exerts on the table, equal in magnitude to $\vec{F_s}$ but opposite in direction (so the force the block exerts on the table ends up being in the same direction as the force from your hand).