Digital Read Serial tutorial - calculating the resistor value The Arduino Digital Read Serial tutorial guides you through the wiring of a simple button. My questions are centered around the resistor.

Why is the resistor necessary?

I think I understand this, but correct me if I am wrong. To ensure that the input pin reads a low signal, and does not float, we should connect the pin to ground when the button is not pushed. When the button gets pushed, we will have a 5V signal flowing to both the pin and to ground. Kirchhoff's voltage law states that total voltage in closed loop must equal zero, so we have to deal with this 5V that is flowing directly to ground. That is why we put a resistor between the button and ground.

How was the resistor value chosen?

This is what I am unclear on. Ohm's law states that...

resistance = voltage / current


This is a 5V supply and the Arduino Uno spec says that the pin provides 40 mA of DC current. So..

resistance = 5V / 0.04A = 125 Ohms


In the Digital Read Serial tutorial, why do we have a 10K Ohms resistor? That is way overkill, right? Which leads to another question: Can you ever put too much resistance in place, if your only goal is to obliterate voltage?

Thanks in advance for any feedback!
You're very close. The resistor is there to act as a pull down resistor; If the input pin is floating it is just an antenna picking up random static. The key is that when positive is supplied (by pressing the button) the positive source needs to "overwhelm" the pull down resistor so that the input pin reads positive. 

If there were no resistance, than the input pin would be reading in the middle of a direct path that does not favor positive or negative. Asside from the direct short that would be occurring, Which state the arduino would read I suspect would either be random or some property of the power supply; It would probably still read ground. 

With the resistor, the signal is read directly connected to positive, before the load resistance. This way it will be sure to get a HIGH signal. 

The exact size of the resistor is of little consequence. Too small and you waste current, but too much resistance is effectively just disconnecting ground again. There is a very wide range of values that will work fine between those. 

I would like to note that when an arduino pin is set to input mode, it will effectively supply no current. It can be moddeled is being in series with a 100 megaOhm resistor. see http://arduino.cc/en/Tutorial/DigitalPins 
for details.