So we have a barrier, the hydrophobic membrane, that prevents the sodium ion from actually moving down its potential gradient. In this case, we have gravitational potential gradient. So from a high gravitational to a low gravitation. But it can't move because of the presence of this barrier, the hydrophobic core. So what we have happening is we have another force that points in the opposite direction, the force due to the presence of this hydrophobic barrier. Let's call that simply FH.