What is the action force and the reaction force you sit down on a chair

Explanation: By sitting down on a chair, we apply weight (w = mg) on the chair so our body weight is the action force. As we know by newton's third law of motion chair will also apply an opposite force equal to our body weight on us. This opposite force applied by chair on us is the reaction force.

Action force is your weight pushing down on the chair. The reaction force is the force exerted by the chair that pushes up on your body and is equal to your weight. objects have inertia.