We have an ideal gas enclosed in cylinder whose top is covered by a piston of certain weight $mg$. At this stage, the piston is at rest which means the force by which the gas acts on the piston ($F=PA$ where $P$ is the gas pressure and $A$ is the cross sectional area of the cylinder) is equal to $mg$.
Now if this system was subject to an isobaric process, then its temperature and volume change with its pressure held constant(suppose $T$ and $V$ increase). But this is confusing, since during the process the piston is continuously changing its position, which implies it's being acted upon by a certain net force; Now the weight of the piston $mg$ is constant, so the force $F=PA$ by which the gas acts on the piston must have increased, but since $A$ is constant, therefore $P$ must have increased; therefore $P$ is not constant.
So How isobaric process is physically possible?