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We can approximate the force of air resistance is …"> <meta name="apple-mobile-web-app-capable" content="yes"> <meta name="mobile-web-app-capable" content="yes"> <meta name="apple-mobile-web-app-status-bar-style" content="default"> <link rel="manifest" href="./manifest.json"><meta property="og:title" content="" /> <meta property="og:description" content="#start of lecture 1 Intro (Newton example): Newton example where we find the equations to describe a falling object using differential equations (DE’s) We know $F=ma$ $F=m\frac{dv}{dt}=mg-kv$ <- we account for air resistance here. We can approximate the force of air resistance is proportional to the speed times a constant. We can rearrange and solve it as it is a separable DE: $\frac{dv}{mg-kv}=\frac{dt}{m}$ integrating both sides: $\int \frac{dv}{mg-kv}=\frac{t}{m}+C$ let $u=mg-kv \quad du=-kdv$ $\int \frac{dv}{mg-kv}=\int \frac{du}{-k*u}=\frac{1}{-k}\ln\mid mg-kv\mid=\frac{t}{m}+C$ Very cool, but I want the velocity as a function of time, isolate v $\ln\mid mg-kv\mid=-\frac{kt}{m}+C$ $\mid mg-kv\mid=e^{\frac{-kt}{m}+C}=e^{\frac{-kt}{m}}e^C$ $e^C$ is a + constant, the absolute value will multiply the inside expression by -1 when the inside is negative, so we can replace the $e^C$ constant with an arbitrary constant A that can be + or - $mg-kv=Ae^{\frac{-kt}{m}}$ so, the general solution is $v(t)=\frac{1}{k}(mg-Ae^{\frac{-kt}{m}})$" /> <meta property="og:type" content="article" /> <meta property="og:url" content="/seperable-de-lec-1.html" /> <meta name="twitter:card" content="summary"/> <meta name="twitter:title" content=""/> <meta name="twitter:description" content="#start of lecture 1 Intro (Newton example): Newton example where we find the equations to describe a falling object using differential equations (DE’s) We know $F=ma$ $F=m\frac{dv}{dt}=mg-kv$ <- we account for air resistance here. We can approximate the force of air resistance is proportional to the speed times a constant. We can rearrange and solve it as it is a separable DE: $\frac{dv}{mg-kv}=\frac{dt}{m}$ integrating both sides: $\int \frac{dv}{mg-kv}=\frac{t}{m}+C$ let $u=mg-kv \quad du=-kdv$ $\int \frac{dv}{mg-kv}=\int \frac{du}{-k*u}=\frac{1}{-k}\ln\mid mg-kv\mid=\frac{t}{m}+C$ Very cool, but I want the velocity as a function of time, isolate v $\ln\mid mg-kv\mid=-\frac{kt}{m}+C$ $\mid mg-kv\mid=e^{\frac{-kt}{m}+C}=e^{\frac{-kt}{m}}e^C$ $e^C$ is a + constant, the absolute value will multiply the inside expression by -1 when the inside is negative, so we can replace the $e^C$ constant with an arbitrary constant A that can be + or - $mg-kv=Ae^{\frac{-kt}{m}}$ so, the general solution is $v(t)=\frac{1}{k}(mg-Ae^{\frac{-kt}{m}})$"/> <title>Seperable DE (lec 1) - My New Hugo Site</title> <link rel="stylesheet" href="./css/main.min.203106d73d4370d04c60441691746dd8e021e38bbbc83f65f636dc8ae886a9f3.css" /> <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script> <script src="./js/main.min.2dd2f7073384163751d1886bcb921097bc2af8ec60cb37deebf49f61a0eca5c3.js" integrity="sha256-LdL3BzOEFjdR0Yhry5IQl7wq+Oxgyzfe6/SfYaDspcM="></script> </head> <body> <style> search-menu { display: block; 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We can approximate the force of air resistance is proportional to the speed times a constant. We can rearrange and solve it as it is a separable DE: $\frac{dv}{mg-kv}=\frac{dt}{m}$ integrating both sides: $\int \frac{dv}{mg-kv}=\frac{t}{m}+C$ let $u=mg-kv \quad du=-kdv$ $\int \frac{dv}{mg-kv}=\int \frac{du}{-k*u}=\frac{1}{-k}\ln\mid mg-kv\mid=\frac{t}{m}+C$ Very cool, but I want the velocity as a function of time, isolate v $\ln\mid mg-kv\mid=-\frac{kt}{m}+C$ $\mid mg-kv\mid=e^{\frac{-kt}{m}+C}=e^{\frac{-kt}{m}}e^C$ $e^C$ is a + constant, the absolute value will multiply the inside expression by -1 when the inside is negative, so we can replace the $e^C$ constant with an arbitrary constant A that can be + or - $mg-kv=Ae^{\frac{-kt}{m}}$ so, the general solution is $v(t)=\frac{1}{k}(mg-Ae^{\frac{-kt}{m}})$</p> <h3 id="separable-de">Separable DE:</h3> <pre><code>$\frac{dy}{dx}=f(y)g(x) \rightarrow \frac{dy}{f(y)}=g(x)dx\quad where\quad f(y)\ne0$ (I'm calling this <a class="hashtag" onclick="focusTag(this)">de_s_type1)</a> </code></pre> <p>ex: $\frac{dy}{dt}=\frac{1-t^2}{y^2}$ $y^2dy=dt(1-t^2)$ integrating both sides yields: $\frac{y^3}{3}=t-\frac{t^3}{3}+C$ $y=(3t-t^3+C)^\frac{1}{3}$</p> <h3 id="initial-value-problem-ivp">Initial value problem (IVP):</h3> <pre><code>A Differential equation with provided initial conditions. </code></pre> <p><a class="hashtag" onclick="focusTag(this)">ex</a> <a class="hashtag" onclick="focusTag(this)">IVP</a> <a class="hashtag" onclick="focusTag(this)">de_s_type1</a> ex: $\frac{dy}{dx}=2x\cos^2(y), \quad y(0)=\frac{\pi}{4}$ $\frac{dy}{\cos^2(y)}=2xdx$ integrate both sides yields: $\int \frac{dy}{\cos^2(y)}=\tan(y)+C=x^2$ plug in $y(0)=\frac{\pi}{4}$ $\tan\left( \frac{\pi}{4} \right)+C=0$ $1+C=0$ $C=-1$ So, the answer is: $y=\arctan(x^2+1)$</p> <p><a class="hashtag" onclick="focusTag(this)">end</a> of Lecture 1</p> <h3>Referenced in</h3> <ul> <li>No backlinks found</li> </ul> </div> </main> <script type="text/javascript"> </script> </body> </html>