Working out the voltage across a second resistor in a series circuit?

Since this is a series circuit, both resistors have the same current I flowing through them. Then conservation of energy requires:

V = IR1 + IR2 -->> I = V/(R1 + R2) = 5V/(6 + 4 Ohms) = 1/2 Amp

The voltae across R2 is simply V2 = IR2 = 0.5A*4Ohms = 2 V

If you short R1 then the voltage across R2 is that of the battery, 5V