* Air is much less dense than a solid object: When you punch a solid object, like a wall, the force you exert is transferred back to you. The wall doesn't move much, so your hand recoils. However, air is very light and moves easily. When you punch the air, you're pushing on a large volume of air, but the air disperses the force widely.
* Force is spread over a large area: The air resistance you feel is spread out over your entire hand and arm. The force is not concentrated in one point like it is when you punch a solid object.
* Your body absorbs the force: Your muscles and bones are designed to absorb impact. The force of the punch is dissipated through your body, making the reaction less noticeable.
In summary: The equal and opposite reaction is still there when you punch the air, but it's much less noticeable because:
* Air is less dense than a solid object, so the force is spread out over a wider area.
* Your body absorbs much of the force.
Let me know if you have any other physics questions!
