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RAJASTHAN ­ PET Rajasthan PET Solved Paper-2003

  • question_answer
    A body cools down from\[61{}^\circ C\]to\[59{}^\circ C\]in 4 min. If the temperature of atmosphere is\[30{}^\circ C,\]then the time taken to cool it from\[51{}^\circ C\]to\[49{}^\circ C\]will be

    A)  4 min           

    B)  2 min

    C)  6 min           

    D)  8 min

    Correct Answer: C

    Solution :

     According to Newton's cooling law, \[\frac{{{\theta }_{1}}-{{\theta }_{2}}}{t}=K\left( \frac{{{\theta }_{1}}+{{\theta }_{2}}}{2}-\theta  \right)\] \[\therefore \] \[\frac{61-59}{4}=K\left( \frac{61+59}{2}-30 \right)\] \[\Rightarrow \] \[K=\frac{1}{60}\] \[\frac{51-49}{t}=\frac{1}{60}\left( \frac{51+49}{2}-30 \right)\] \[t=6\text{ }min.\]


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