In the context you provided, where you are sitting in a chair and not moving relative to the ground but still feeling the sensation of gravity, it's important to understand the concept of acceleration and its relationship to movement.
Acceleration is defined as a change in velocity over time. Velocity, in turn, is a vector quantity that describes both speed and direction. When you are sitting in a chair, even though you may not be changing your position relative to the ground (i.e., your speed is constant), you are still experiencing acceleration due to gravity.
Gravity is the force that attracts objects with mass toward each other. On Earth, gravity pulls everything toward the center of the planet. When you are sitting in a chair, the chair and your body are being pulled downward by gravity. Even though you are not changing your position relative to the ground, there is still an acceleration acting upon you due to gravity.
This sensation of acceleration due to gravity is commonly referred to as the feeling of weight. It is the force exerted by the chair on your body to counteract the downward force of gravity. According to Newton's second law of motion, when a force acts on an object, it causes an acceleration. In this case, the force of the chair pushing against your body counteracts the downward force of gravity, resulting in an acceleration of zero (since you are not moving relative to the ground).
So, while you may not be physically moving, you can still feel the effects of gravity as an acceleration. The sensation of weight you experience while sitting in the chair is due to the counteracting forces acting on your body.