When an object reaches its terminal velocity while falling through a planet's atmosphere, the force of gravity and the force of air resistance acting on the object are in equilibrium. Terminal velocity is the maximum velocity that the object can attain during free fall when the drag force of air resistance becomes equal in magnitude and opposite in direction to the force of gravity.
Gravity continues to act on the object even at terminal velocity. However, since the drag force from air resistance is equal to the force of gravity, there is no net acceleration on the object, and it falls at a constant velocity. This constant velocity is known as the terminal velocity.
The mass of the object remains the same as it falls through the atmosphere. Mass is a measure of the amount of matter an object contains, and it remains constant regardless of its location or motion.
Weight, on the other hand, can change as the object falls through the planet's atmosphere. Weight is the force experienced by an object due to gravity and is directly proportional to the mass of the object. As the object falls through the atmosphere, it experiences a reduction in weight due to the opposing force of air resistance. The greater the air resistance, the more the weight of the object is offset.
As the object falls from a higher altitude, the atmosphere becomes denser, and the air resistance increases. This causes a more significant reduction in the net force acting on the object, leading to a decrease in weight. However, once the object reaches terminal velocity, the reduction in weight due to air resistance is balanced by the force of gravity, resulting in a constant weight throughout its descent.