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 Homework Statement

A pendulum, with a block of iron (##m=1 kg##, specific heat ##c=448\frac{J}{Kg\cdot K}##) hanging by a thread (mass negligible), is inside a container of volume with rigid and adiabatic walls. Inside the container there's air (to consider as a biatomic ideal gas) at the atmospheric pressure and at the temperature ##T_0=300 K##. You observe the pendulum has a speed of ##v=3.5 \frac{m}{s}## when the block is on the vertical. After some time, the pendulum ceases to swing. Determine:
a) The final temperature of the system at the end of the oscillation.
b) The variation of entropy of the universe at the end of the trasformation.
 Homework Equations
 ##Q=m*c*(T_fT_0)##
The kinetic energy of the pendulum ##K=\frac{1}{2}\cdot m\cdot v^2## will turn into heat (entirely).
So both the air and the block of iron will change their temperature.
To find ##n## (moles of the gas) I can use the ideal gas law:
##n=\frac{pV}{RT}=0.9 mol##
Do I have the following equation?
##\frac{1}{2}mv^2=m*c*(T_fT_0)+n*c_v*(T_fT_0)## and ##T_f## is my unknown.
But I get ##T_f=300K## (nearly). Where's my mistake?
So both the air and the block of iron will change their temperature.
To find ##n## (moles of the gas) I can use the ideal gas law:
##n=\frac{pV}{RT}=0.9 mol##
Do I have the following equation?
##\frac{1}{2}mv^2=m*c*(T_fT_0)+n*c_v*(T_fT_0)## and ##T_f## is my unknown.
But I get ##T_f=300K## (nearly). Where's my mistake?