Confusion about how KCL and KVL are used for diode circuits

[timnswede]

Homework Statement

This is just one of the example problems in my book which is already solved for me, but I don't really understand their solution which I'll post here:
They start out with assuming Vin is very negative, which makes D1 turn on and makes Vout=VD,on+Vin. THen they solve for the currents.

Homework Equations

KCL/KVL

The Attempt at a Solution

What I am confused on is that if the diode is turned on, then the branch with the diode is a short circuit, so why is there current going in the R2 branch?
The book solves all the problems like this one the same way, but I don't really get it. Are we assuming the diode is actually on the brink of turning on/off, so it allows current through it, but it's actually not a short circuit yet?

 

[gneill]

What I am confused on is that if the diode is turned on, then the branch with the diode is a short circuit, so why is there current going in the R2 branch?

What does V_D.on represent? Is the diode treated as ideal or non-ideal in these problems?

 

[cnh1995]

if the diode is turned on, then the branch with the diode is a short circuit,

No. You are asked to use the constant voltage model for the diode.

 

[timnswede]

Oh wow, not sure why I didn't realize it until you guys said it, but now it makes sense, thanks!