# 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

## 1 Answer

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).

There are always force pairs, but they are acting on different objects. You don't always need to consider the "other" member of the pair. In fact, in a proper free-body diagram (for a single object), you will not show the other member of the pair.

• Thank you very much Bill N for spending time to answer my question. I am a biologist so your answer was so much informative to me. You state that "The 3rd law partner force is a force which the block exerts on the table". What is the origin of this force? Isn't this the force that I apply on the block? – Kosala Sep 13 '17 at 23:05
• I could ask "What is the origin of the force which the table exerts on the block?" The answer to both your question and mine is the interaction of the roughness (bumps and grooves) of both surfaces hitting each other and resisting the sliding of the surfaces. No, the force you apply to the block acts at the point of contact between your hand and the block. The friction force might change because your hand causes the block and table roughness to interact more strongly, but I would call this a consequence rather than an origin. Remember that Newton's laws are part of building a model. – Bill N Sep 14 '17 at 1:04
• Thanks Bill. Still need to get clarified something. I will post it as a question. So others can learn something. The topic will be "What would be the counter force". – Kosala Sep 14 '17 at 16:20