net aerodynamic force will always have the opposite direction to the movement of a rocket.
This is certainly not true in general, this is to ignore lift. Lift is the ability of some shapes to bend the net aerodynamic force away from being directly opposite the motion. Planes rely heavily on this.
However, as a modelling choice, ignoring lift and calculating drag only is a reasonable choice as simple way of approximating aerodynamics for a model rocket.
if the rocket's angle of attack is 0 (It's moving perpendicular to the Earth) would the net aerodynamic force be equal to drag?
Angle of attack is not a measure of the absolute direction of movement, it's the angle between an object's attitude and its relative velocity to the air.
Going straight up, something could actually have any attitude, and thus any angle of attack.
But with some extra assumptions the statement would be true:
- The rocket is also pointing straight up, in addition to going straight up.
- The rocket is symmetric along the axis.
All of this is usually approximately the case for a model rocket, so yes, the net aerodynamic force is usually equal to drag, and the lift would be close to zero.
would Drag be equal to the net force times cosα being α the angle of atack? And would Lift be equal to the net force times sinα?
No. The direction of net force is not directly tied to angle of attack.
A way of visualising this is that a plane with a 90 degree angle of attack, going with its belly first into the wind, does not have perfect lift.