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A coffee with temperature \(\displaystyle 171^\circ\) is left in a room with temperature \(\displaystyle 55^\circ\). After 5 minutes the temperature of the coffee is \(\displaystyle 165^\circ\), what is the temperature of the coffee after 14 minutes?
Newton's law of Cooling states that the change in temperature is directly proportional to the difference between the object's temperature and its surroundings. \begin{equation*} \frac{dT}{dt} = k(T(t)-T_{\text{room}})\end{equation*}Using the substitution \(\displaystyle y(t)=T(t)-55\) and calculating the derivative gives \(\displaystyle \frac{dy}{dt}=\frac{dT}{dt}\). Calculating the new initial condition using the point \(\displaystyle (5, 165)\) and the substition gives \(\displaystyle y(0) = T(0)-55 = 116\). The point \(\displaystyle (5, 165)\) must also be transformed to get \(\displaystyle y(5) = T(5)-55 = 165 - 55 = 110\). Substituting both of these into the equation gives the new equaiton \(\displaystyle \frac{dy}{dt}=ky\) which has the solution \(\displaystyle y(t) = y(0)e^{kt}=116e^{kt}\). Evaluating the function at the point gives \(\displaystyle 110=116e^{5k}\) and isolating the exponential gives \(\displaystyle \frac{55}{58}=e^{5k}\). Solving for \(\displaystyle k\) gives \(\displaystyle k=\frac{\ln{\left(\frac{55}{58} \right)}}{5}\). Substuting \(\displaystyle k\) back into the equation gives \(\displaystyle y(t) = 116e^{\frac{\ln{\left(\frac{55}{58} \right)}}{5}t}\) and simplifying gives \(\displaystyle y(t) = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}}\). Substituting out \(\displaystyle y(t)\) gives \begin{equation*}T(t)-55 = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} \implies\, T(t)= 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} + 55 \end{equation*}Using \(\displaystyle t = 14\) gives \(\displaystyle T =116 \left(\frac{55}{58}\right)^{\frac{14}{5}} + 55\approx 155.00^\circ\)
\begin{question}A coffee with temperature $171^\circ$ is left in a room with temperature $55^\circ$. After 5 minutes the temperature of the coffee is $165^\circ$, what is the temperature of the coffee after 14 minutes? \soln{9cm}{Newton's law of Cooling states that the change in temperature is directly proportional to the difference between the object's temperature and its surroundings. \begin{equation*} \frac{dT}{dt} = k(T(t)-T_{\text{room}})\end{equation*}Using the substitution $y(t)=T(t)-55$ and calculating the derivative gives $\frac{dy}{dt}=\frac{dT}{dt}$. Calculating the new initial condition using the point $(5, 165)$ and the substition gives $y(0) = T(0)-55 = 116$. The point $(5, 165)$ must also be transformed to get $y(5) = T(5)-55 = 165 - 55 = 110$. Substituting both of these into the equation gives the new equaiton $\frac{dy}{dt}=ky$ which has the solution $y(t) = y(0)e^{kt}=116e^{kt}$. Evaluating the function at the point gives $110=116e^{5k}$ and isolating the exponential gives $\frac{55}{58}=e^{5k}$. Solving for $k$ gives $k=\frac{\ln{\left(\frac{55}{58} \right)}}{5}$. Substuting $k$ back into the equation gives $y(t) = 116e^{\frac{\ln{\left(\frac{55}{58} \right)}}{5}t}$ and simplifying gives $y(t) = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}}$. Substituting out $y(t)$ gives \begin{equation*}T(t)-55 = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} \implies\, T(t)= 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} + 55 \end{equation*}Using $t = 14$ gives $T =116 \left(\frac{55}{58}\right)^{\frac{14}{5}} + 55\approx 155.00^\circ$} \end{question}
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<p> <p>A coffee with temperature <img class="equation_image" title=" \displaystyle 171^\circ " src="/equation_images/%20%5Cdisplaystyle%20171%5E%5Ccirc%20" alt="LaTeX: \displaystyle 171^\circ " data-equation-content=" \displaystyle 171^\circ " /> is left in a room with temperature <img class="equation_image" title=" \displaystyle 55^\circ " src="/equation_images/%20%5Cdisplaystyle%2055%5E%5Ccirc%20" alt="LaTeX: \displaystyle 55^\circ " data-equation-content=" \displaystyle 55^\circ " /> . After 5 minutes the temperature of the coffee is <img class="equation_image" title=" \displaystyle 165^\circ " src="/equation_images/%20%5Cdisplaystyle%20165%5E%5Ccirc%20" alt="LaTeX: \displaystyle 165^\circ " data-equation-content=" \displaystyle 165^\circ " /> , what is the temperature of the coffee after 14 minutes?</p> </p>
<p> <p>Newton's law of Cooling states that the change in temperature is directly proportional to the difference between the object's temperature and its surroundings. <img class="equation_image" title=" \frac{dT}{dt} = k(T(t)-T_{\text{room}}) " src="/equation_images/%20%20%5Cfrac%7BdT%7D%7Bdt%7D%20%3D%20k%28T%28t%29-T_%7B%5Ctext%7Broom%7D%7D%29%20" alt="LaTeX: \frac{dT}{dt} = k(T(t)-T_{\text{room}}) " data-equation-content=" \frac{dT}{dt} = k(T(t)-T_{\text{room}}) " /> Using the substitution <img class="equation_image" title=" \displaystyle y(t)=T(t)-55 " src="/equation_images/%20%5Cdisplaystyle%20y%28t%29%3DT%28t%29-55%20" alt="LaTeX: \displaystyle y(t)=T(t)-55 " data-equation-content=" \displaystyle y(t)=T(t)-55 " /> and calculating the derivative gives <img class="equation_image" title=" \displaystyle \frac{dy}{dt}=\frac{dT}{dt} " src="/equation_images/%20%5Cdisplaystyle%20%5Cfrac%7Bdy%7D%7Bdt%7D%3D%5Cfrac%7BdT%7D%7Bdt%7D%20" alt="LaTeX: \displaystyle \frac{dy}{dt}=\frac{dT}{dt} " data-equation-content=" \displaystyle \frac{dy}{dt}=\frac{dT}{dt} " /> . Calculating the new initial condition using the point <img class="equation_image" title=" \displaystyle (5, 165) " src="/equation_images/%20%5Cdisplaystyle%20%285%2C%20165%29%20" alt="LaTeX: \displaystyle (5, 165) " data-equation-content=" \displaystyle (5, 165) " /> and the substition gives <img class="equation_image" title=" \displaystyle y(0) = T(0)-55 = 116 " src="/equation_images/%20%5Cdisplaystyle%20y%280%29%20%3D%20T%280%29-55%20%3D%20116%20" alt="LaTeX: \displaystyle y(0) = T(0)-55 = 116 " data-equation-content=" \displaystyle y(0) = T(0)-55 = 116 " /> . The point <img class="equation_image" title=" \displaystyle (5, 165) " src="/equation_images/%20%5Cdisplaystyle%20%285%2C%20165%29%20" alt="LaTeX: \displaystyle (5, 165) " data-equation-content=" \displaystyle (5, 165) " /> must also be transformed to get <img class="equation_image" title=" \displaystyle y(5) = T(5)-55 = 165 - 55 = 110 " src="/equation_images/%20%5Cdisplaystyle%20y%285%29%20%3D%20T%285%29-55%20%3D%20165%20-%2055%20%3D%20110%20" alt="LaTeX: \displaystyle y(5) = T(5)-55 = 165 - 55 = 110 " data-equation-content=" \displaystyle y(5) = T(5)-55 = 165 - 55 = 110 " /> . Substituting both of these into the equation gives the new equaiton <img class="equation_image" title=" \displaystyle \frac{dy}{dt}=ky " src="/equation_images/%20%5Cdisplaystyle%20%5Cfrac%7Bdy%7D%7Bdt%7D%3Dky%20" alt="LaTeX: \displaystyle \frac{dy}{dt}=ky " data-equation-content=" \displaystyle \frac{dy}{dt}=ky " /> which has the solution <img class="equation_image" title=" \displaystyle y(t) = y(0)e^{kt}=116e^{kt} " src="/equation_images/%20%5Cdisplaystyle%20y%28t%29%20%3D%20y%280%29e%5E%7Bkt%7D%3D116e%5E%7Bkt%7D%20" alt="LaTeX: \displaystyle y(t) = y(0)e^{kt}=116e^{kt} " data-equation-content=" \displaystyle y(t) = y(0)e^{kt}=116e^{kt} " /> . Evaluating the function at the point gives <img class="equation_image" title=" \displaystyle 110=116e^{5k} " src="/equation_images/%20%5Cdisplaystyle%20110%3D116e%5E%7B5k%7D%20" alt="LaTeX: \displaystyle 110=116e^{5k} " data-equation-content=" \displaystyle 110=116e^{5k} " /> and isolating the exponential gives <img class="equation_image" title=" \displaystyle \frac{55}{58}=e^{5k} " src="/equation_images/%20%5Cdisplaystyle%20%5Cfrac%7B55%7D%7B58%7D%3De%5E%7B5k%7D%20" alt="LaTeX: \displaystyle \frac{55}{58}=e^{5k} " data-equation-content=" \displaystyle \frac{55}{58}=e^{5k} " /> . Solving for <img class="equation_image" title=" \displaystyle k " src="/equation_images/%20%5Cdisplaystyle%20k%20" alt="LaTeX: \displaystyle k " data-equation-content=" \displaystyle k " /> gives <img class="equation_image" title=" \displaystyle k=\frac{\ln{\left(\frac{55}{58} \right)}}{5} " src="/equation_images/%20%5Cdisplaystyle%20k%3D%5Cfrac%7B%5Cln%7B%5Cleft%28%5Cfrac%7B55%7D%7B58%7D%20%5Cright%29%7D%7D%7B5%7D%20" alt="LaTeX: \displaystyle k=\frac{\ln{\left(\frac{55}{58} \right)}}{5} " data-equation-content=" \displaystyle k=\frac{\ln{\left(\frac{55}{58} \right)}}{5} " /> . Substuting <img class="equation_image" title=" \displaystyle k " src="/equation_images/%20%5Cdisplaystyle%20k%20" alt="LaTeX: \displaystyle k " data-equation-content=" \displaystyle k " /> back into the equation gives <img class="equation_image" title=" \displaystyle y(t) = 116e^{\frac{\ln{\left(\frac{55}{58} \right)}}{5}t} " src="/equation_images/%20%5Cdisplaystyle%20y%28t%29%20%3D%20116e%5E%7B%5Cfrac%7B%5Cln%7B%5Cleft%28%5Cfrac%7B55%7D%7B58%7D%20%5Cright%29%7D%7D%7B5%7Dt%7D%20" alt="LaTeX: \displaystyle y(t) = 116e^{\frac{\ln{\left(\frac{55}{58} \right)}}{5}t} " data-equation-content=" \displaystyle y(t) = 116e^{\frac{\ln{\left(\frac{55}{58} \right)}}{5}t} " /> and simplifying gives <img class="equation_image" title=" \displaystyle y(t) = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} " src="/equation_images/%20%5Cdisplaystyle%20y%28t%29%20%3D%20116%20%5Cleft%28%5Cfrac%7B55%7D%7B58%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B5%7D%7D%20" alt="LaTeX: \displaystyle y(t) = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} " data-equation-content=" \displaystyle y(t) = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} " /> . Substituting out <img class="equation_image" title=" \displaystyle y(t) " src="/equation_images/%20%5Cdisplaystyle%20y%28t%29%20" alt="LaTeX: \displaystyle y(t) " data-equation-content=" \displaystyle y(t) " /> gives
<img class="equation_image" title=" T(t)-55 = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} \implies\, T(t)= 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} + 55 " src="/equation_images/%20T%28t%29-55%20%3D%20116%20%5Cleft%28%5Cfrac%7B55%7D%7B58%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B5%7D%7D%20%5Cimplies%5C%2C%20T%28t%29%3D%20116%20%5Cleft%28%5Cfrac%7B55%7D%7B58%7D%5Cright%29%5E%7B%5Cfrac%7Bt%7D%7B5%7D%7D%20%2B%2055%20%20" alt="LaTeX: T(t)-55 = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} \implies\, T(t)= 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} + 55 " data-equation-content=" T(t)-55 = 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} \implies\, T(t)= 116 \left(\frac{55}{58}\right)^{\frac{t}{5}} + 55 " /> Using <img class="equation_image" title=" \displaystyle t = 14 " src="/equation_images/%20%5Cdisplaystyle%20t%20%3D%2014%20" alt="LaTeX: \displaystyle t = 14 " data-equation-content=" \displaystyle t = 14 " /> gives <img class="equation_image" title=" \displaystyle T =116 \left(\frac{55}{58}\right)^{\frac{14}{5}} + 55\approx 155.00^\circ " src="/equation_images/%20%5Cdisplaystyle%20T%20%3D116%20%5Cleft%28%5Cfrac%7B55%7D%7B58%7D%5Cright%29%5E%7B%5Cfrac%7B14%7D%7B5%7D%7D%20%2B%2055%5Capprox%20155.00%5E%5Ccirc%20" alt="LaTeX: \displaystyle T =116 \left(\frac{55}{58}\right)^{\frac{14}{5}} + 55\approx 155.00^\circ " data-equation-content=" \displaystyle T =116 \left(\frac{55}{58}\right)^{\frac{14}{5}} + 55\approx 155.00^\circ " /> </p> </p>