Hello Vennapusa charan kumar reddy,

Well, at the terminal velocity, its velocity will not change, hence its momentum will also not change and therefore rate of change of momentum is 0 or the accelaration will be 0 (since accelaration = rate of change of momentum ), so: At the terminal velocity,

Fnet=mg−FD=ma=0.(where FD is the drift force due to air resistance, say FD = 1/2CρAv^2 )(where C is constant, ρ is the density of air, A is the surface area of the body, V is the velocity of body and in this case V = Vt or terminal velocity )

Thus,

mg=FD.mg=FD.

Using the equation for drag force, we have

mg=1/2CρAv^2

Solving for the velocity, we obtain

vt=√2mgρCA.

for more details you can refer to ( Terminal Velocity (https://courses.lumenlearning.com/suny-osuniversityphysics/chapter/6-4-drag-force-and-terminal-speed/) )

Hope this helps, and feel free to ask any further query...