Question

the reaction force of a chair you are sitting on

Answer 1

That reaction force is the normal force the chair exerts on you.

Key points
- The normal force is a contact force acting perpendicular to the chair’s surface, upward on you when you sit.
- If you are at rest (no vertical acceleration), the normal force N equals your weight: N = mg (m = your mass, g ≈ 9.81 m/s²). Example: a 70 kg person → N ≈ 70×9.81 ≈ 687 N.
- If you accelerate vertically, N changes. From Newton’s 2nd law (up positive): N − mg = ma, so N = m(g + a). (If you accelerate downward a is negative.)
- By Newton’s 3rd law, the chair’s upward normal force on you has an equal and opposite partner: you exert a downward force of the same magnitude on the chair. (The gravitational force on you and the normal force are not an action–reaction pair; the action pair to gravity is the pull you exert on Earth.)

Other notes
- On a real chair the normal force is distributed among the seat and legs and transmitted to the floor.
- If you lean or shift, the weight distribution and normal forces on different parts change and torques may arise.