Updated spacing between @ latex expressions
en/7.1.22.md
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| Solutions of Savchenko Problems in Physics <br> | |||
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| <h3> | |||
| Problem statement: | |||
| </h3> | |||
| <div class="task"> | |||
| <p> | |||
| \(7.1.22^*\) | |||
| Find the frequency of small oscillations of the mathematical pendulum relative to its lower equilibrium position, if a charge $Q$ is fixed directly under the equilibrium position of the ball at a distance $h$ from it. Length of the thread $l$, mass of the ball $m$, charge $q$. <br> | |||
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| <h3> | |||
| Solution: | |||
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| <div class="solutions" > | |||
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| <br> | |||
| <p class="TxtSolutions"> | |||
| Applying Newton's Second Law: | |||
| @@ -92,7 +92,7 @@ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| − | $mg\sin{\theta}-\frac{kqQ}{\left[h+l(1-\cos\theta)\right]^2+l^2\sin^2 {\theta}}\sin{(\beta+\theta)} $$= m\frac{dv}{dt} \; (1)$ | ||
| + | $mg\sin{\theta}-\frac{kqQ}{\left[h+l(1-\cos\theta )\right]^2+l^2\sin^2 {\theta}}\sin{(\beta +\theta )} $$= m\frac{dv}{dt} \; (1)$ | ||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| where, | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| $\tan{\beta} = \frac{l\sin{\theta}}{h+l\left(1-\cos\theta\right)} \; (2)$ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| and, it is known that | |||
| @@ -108,15 +108,15 @@ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| − | $\sin{(\beta+\theta)} = \sin{\beta}\cos{\theta}+\cos{\beta}\sin{\theta}$ | ||
| + | $\sin{(\beta +\theta )} = \sin{\beta}\cos{\theta}+\cos{\beta}\sin{\theta}$ | ||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| as $\theta$ is small (small oscillations), $\sin{\theta}\approx\theta$ and $\cos{\theta}\approx1-\theta^2/2$, then | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| − | $\sin{(\beta+\theta)} = \sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta} \label{E3} \; (3)$ | ||
| + | $\sin{(\beta +\theta )} = \sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta} \label{E3} \; (3)$ | ||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| Modifying $(2)$, | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| $\tan{\beta} = \frac{l\theta}{h+\frac{l\theta^2}{2}} \; (4)$ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| Substituting $(3)$ and $(4)$ into $(1)$ and taking account $\sin{\theta}\approx\theta$ | |||
| @@ -132,15 +132,15 @@ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| − | $mg\theta-\frac{kqQ}{\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2}\left[\sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta}\right] = m\frac{dv}{dt} \label{NSLmod}$ | ||
| + | $mg\theta -\frac{kqQ}{\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2}\left[\sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta}\right] = m\frac{dv}{dt} \label{NSLmod}$ | ||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| Since angular acceleration is related to lineal acceleration according $dv/dt = \ddot{\theta}l$, dividing all equation by $ml$ and take out $\cos{\beta}$ from second term of left side, it is obtained | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| − | $\frac{g}{l}\theta-\frac{kqQ\cos{\beta}}{ml\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]}\left[\left(1-\frac{\theta^2}{2}\right)\tan{\beta}+\theta\right] = \ddot{\theta} \label{NEq} \; (5)$ | ||
| + | $\frac{g}{l}\theta -\frac{kqQ\cos{\beta}}{ml\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]}\left[\left(1-\frac{\theta^2}{2}\right)\tan{\beta}+\theta\right] = \ddot{\theta} \label{NEq} \; (5)$ | ||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| Graphically it can be seen that, with cosine and sine approximations applied, | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| $\cos{\beta} = \frac{h+\frac{l\theta^2}{2}}{\sqrt{\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2}}$ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| Putting $(4)$ and $(6)$ into $(5)$ and developing algebraically | |||
| @@ -156,7 +156,7 @@ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| − | $\frac{g}{l}\theta-\frac{kqQ}{ml}\frac{h}{\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]^{\frac{3}{2}}}\left[\left(1+\frac{l}{h+\frac{l\theta^2}{2}}\right)\theta-\frac{l\theta^3}{2h+l\theta^2}\right] = \ddot{\theta}$ | ||
| + | $\frac{g}{l}\theta -\frac{kqQ}{ml}\frac{h}{\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]^{\frac{3}{2}}}\left[\left(1+\frac{l}{h+\frac{l\theta^2}{2}}\right)\theta -\frac{l\theta^3}{2h+l\theta^2}\right] = \ddot{\theta}$ | ||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| Since there are small oscillations, $\theta\rightarrow0$, but terms $\theta^2$ and $\theta^3$ tend to zero faster than $\theta$, so they are negligible respect to $\theta$. | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| $\left[\frac{g}{l}-\frac{kqQ\left(l+h\right)}{mh^3l}\right]\theta = \ddot{\theta}$ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| as $k = \frac{1}{4\pi\epsilon_0}$, | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| $\left[\frac{g}{l}-\frac{qQ\left(l+h\right)}{4\pi\epsilon_0mh^3l}\right]\theta = \ddot{\theta}$ | |||
| </p> | |||
| <br> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| Finally, | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: center;"> | |||
| $\boxed{\omega = \sqrt{\frac{g}{l}-\frac{qQ\left(h+l\right)}{4\pi\epsilon_0mh^3l}}}$ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions"> | |||
| valid for the condition $\frac{qQ(h+l)}{4\pi\epsilon_0h^3} < mg$ | |||
| </p> | |||
| <br> | |||
| <p class="TxtSolutions" style="text-align: right; font-style: italic; font-size: 14;"> | |||
| BSc. Luis Daniel Fernández Quintana<br> | |||
| Physics Department (FCNE)<br> | |||
| Universidad de Oriente, Cuba<br> | |||
| </p> | |||
| </div> | |||
| <h3> | |||
| Answer: $\omega = \sqrt{\frac{g}{l}-\frac{qQ\left(h+l\right)}{4\pi\epsilon_0mh^3l}}$ | |||
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| </a> | </a> | ||
| <p class="author"> | <p class="author"> | ||
| Solutions of Savchenko Problems in Physics <br> | Solutions of Savchenko Problems in Physics <br> | ||
| <i><b>knowledge must be free</b></i> | <i><b>knowledge must be free</b></i> | ||
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| <div class="col caption"> | <div class="col caption"> | ||
| <h2> | <h2> | ||
| <a href="../#1">←Назад</a> | <a href="../#1">←Назад</a> | ||
| </h2> | </h2> | ||
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| <h3> | <h3> | ||
| Problem statement: | Problem statement: | ||
| </h3> | </h3> | ||
| <div class="task"> | <div class="task"> | ||
| <p> | <p> | ||
| \(7.1.22^*\) | \(7.1.22^*\) | ||
| Find the frequency of small oscillations of the mathematical pendulum relative to its lower equilibrium position, if a charge $Q$ is fixed directly under the equilibrium position of the ball at a distance $h$ from it. Length of the thread $l$, mass of the ball $m$, charge $q$. <br> | Find the frequency of small oscillations of the mathematical pendulum relative to its lower equilibrium position, if a charge $Q$ is fixed directly under the equilibrium position of the ball at a distance $h$ from it. Length of the thread $l$, mass of the ball $m$, charge $q$. <br> | ||
| </p> | </p> | ||
| <div class="StatementDiv"> | <div class="StatementDiv"> | ||
| <img class="Statement" src="statement.png" alt="9.1.14"> | <img class="Statement" src="statement.png" alt="9.1.14"> | ||
| </div> | </div> | ||
| </div> | </div> | ||
| <hr> | <hr> | ||
| <h3> | <h3> | ||
| Solution: | Solution: | ||
| </h3> | </h3> | ||
| <div class="solutions" > | <div class="solutions" > | ||
| <img class="ScreenS" src="drawing.png" style="max-width: 250;"> | <img class="ScreenS" src="drawing.png" style="max-width: 250;"> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Applying Newton's Second Law: | Applying Newton's Second Law: | ||
| @@ -92,7 +92,7 @@ | |||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $mg\sin{\theta}-\frac{kqQ}{\left[h+l(1-\cos\theta)\right]^2+l^2\sin^2 {\theta}}\sin{(\beta+\theta)} $$= m\frac{dv}{dt} \; (1)$ | $mg\sin{\theta}-\frac{kqQ}{\left[h+l(1-\cos\theta )\right]^2+l^2\sin^2 {\theta}}\sin{(\beta +\theta )} $$= m\frac{dv}{dt} \; (1)$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| where, | where, | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\tan{\beta} = \frac{l\sin{\theta}}{h+l\left(1-\cos\theta\right)} \; (2)$ | $\tan{\beta} = \frac{l\sin{\theta}}{h+l\left(1-\cos\theta\right)} \; (2)$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| and, it is known that | and, it is known that | ||
| @@ -108,15 +108,15 @@ | |||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\sin{(\beta+\theta)} = \sin{\beta}\cos{\theta}+\cos{\beta}\sin{\theta}$ | $\sin{(\beta +\theta )} = \sin{\beta}\cos{\theta}+\cos{\beta}\sin{\theta}$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| as $\theta$ is small (small oscillations), $\sin{\theta}\approx\theta$ and $\cos{\theta}\approx1-\theta^2/2$, then | as $\theta$ is small (small oscillations), $\sin{\theta}\approx\theta$ and $\cos{\theta}\approx1-\theta^2/2$, then | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\sin{(\beta+\theta)} = \sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta} \label{E3} \; (3)$ | $\sin{(\beta +\theta )} = \sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta} \label{E3} \; (3)$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Modifying $(2)$, | Modifying $(2)$, | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\tan{\beta} = \frac{l\theta}{h+\frac{l\theta^2}{2}} \; (4)$ | $\tan{\beta} = \frac{l\theta}{h+\frac{l\theta^2}{2}} \; (4)$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Substituting $(3)$ and $(4)$ into $(1)$ and taking account $\sin{\theta}\approx\theta$ | Substituting $(3)$ and $(4)$ into $(1)$ and taking account $\sin{\theta}\approx\theta$ | ||
| @@ -132,15 +132,15 @@ | |||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $mg\theta-\frac{kqQ}{\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2}\left[\sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta}\right] = m\frac{dv}{dt} \label{NSLmod}$ | $mg\theta -\frac{kqQ}{\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2}\left[\sin{\beta}\left(1-\frac{\theta^2}{2}\right)+\theta\cos{\beta}\right] = m\frac{dv}{dt} \label{NSLmod}$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Since angular acceleration is related to lineal acceleration according $dv/dt = \ddot{\theta}l$, dividing all equation by $ml$ and take out $\cos{\beta}$ from second term of left side, it is obtained | Since angular acceleration is related to lineal acceleration according $dv/dt = \ddot{\theta}l$, dividing all equation by $ml$ and take out $\cos{\beta}$ from second term of left side, it is obtained | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\frac{g}{l}\theta-\frac{kqQ\cos{\beta}}{ml\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]}\left[\left(1-\frac{\theta^2}{2}\right)\tan{\beta}+\theta\right] = \ddot{\theta} \label{NEq} \; (5)$ | $\frac{g}{l}\theta -\frac{kqQ\cos{\beta}}{ml\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]}\left[\left(1-\frac{\theta^2}{2}\right)\tan{\beta}+\theta\right] = \ddot{\theta} \label{NEq} \; (5)$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Graphically it can be seen that, with cosine and sine approximations applied, | Graphically it can be seen that, with cosine and sine approximations applied, | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\cos{\beta} = \frac{h+\frac{l\theta^2}{2}}{\sqrt{\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2}}$ | $\cos{\beta} = \frac{h+\frac{l\theta^2}{2}}{\sqrt{\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2}}$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Putting $(4)$ and $(6)$ into $(5)$ and developing algebraically | Putting $(4)$ and $(6)$ into $(5)$ and developing algebraically | ||
| @@ -156,7 +156,7 @@ | |||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\frac{g}{l}\theta-\frac{kqQ}{ml}\frac{h}{\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]^{\frac{3}{2}}}\left[\left(1+\frac{l}{h+\frac{l\theta^2}{2}}\right)\theta-\frac{l\theta^3}{2h+l\theta^2}\right] = \ddot{\theta}$ | $\frac{g}{l}\theta -\frac{kqQ}{ml}\frac{h}{\left[\left(h+\frac{l\theta^2}{2}\right)^2+l^2\theta^2\right]^{\frac{3}{2}}}\left[\left(1+\frac{l}{h+\frac{l\theta^2}{2}}\right)\theta -\frac{l\theta^3}{2h+l\theta^2}\right] = \ddot{\theta}$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Since there are small oscillations, $\theta\rightarrow0$, but terms $\theta^2$ and $\theta^3$ tend to zero faster than $\theta$, so they are negligible respect to $\theta$. | Since there are small oscillations, $\theta\rightarrow0$, but terms $\theta^2$ and $\theta^3$ tend to zero faster than $\theta$, so they are negligible respect to $\theta$. | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\left[\frac{g}{l}-\frac{kqQ\left(l+h\right)}{mh^3l}\right]\theta = \ddot{\theta}$ | $\left[\frac{g}{l}-\frac{kqQ\left(l+h\right)}{mh^3l}\right]\theta = \ddot{\theta}$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| as $k = \frac{1}{4\pi\epsilon_0}$, | as $k = \frac{1}{4\pi\epsilon_0}$, | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\left[\frac{g}{l}-\frac{qQ\left(l+h\right)}{4\pi\epsilon_0mh^3l}\right]\theta = \ddot{\theta}$ | $\left[\frac{g}{l}-\frac{qQ\left(l+h\right)}{4\pi\epsilon_0mh^3l}\right]\theta = \ddot{\theta}$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| Finally, | Finally, | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: center;"> | <p class="TxtSolutions" style="text-align: center;"> | ||
| $\boxed{\omega = \sqrt{\frac{g}{l}-\frac{qQ\left(h+l\right)}{4\pi\epsilon_0mh^3l}}}$ | $\boxed{\omega = \sqrt{\frac{g}{l}-\frac{qQ\left(h+l\right)}{4\pi\epsilon_0mh^3l}}}$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions"> | <p class="TxtSolutions"> | ||
| valid for the condition $\frac{qQ(h+l)}{4\pi\epsilon_0h^3} < mg$ | valid for the condition $\frac{qQ(h+l)}{4\pi\epsilon_0h^3} < mg$ | ||
| </p> | </p> | ||
| <br> | <br> | ||
| <p class="TxtSolutions" style="text-align: right; font-style: italic; font-size: 14;"> | <p class="TxtSolutions" style="text-align: right; font-style: italic; font-size: 14;"> | ||
| BSc. Luis Daniel Fernández Quintana<br> | BSc. Luis Daniel Fernández Quintana<br> | ||
| Physics Department (FCNE)<br> | Physics Department (FCNE)<br> | ||
| Universidad de Oriente, Cuba<br> | Universidad de Oriente, Cuba<br> | ||
| </p> | </p> | ||
| </div> | </div> | ||
| <h3> | <h3> | ||
| Answer: $\omega = \sqrt{\frac{g}{l}-\frac{qQ\left(h+l\right)}{4\pi\epsilon_0mh^3l}}$ | Answer: $\omega = \sqrt{\frac{g}{l}-\frac{qQ\left(h+l\right)}{4\pi\epsilon_0mh^3l}}$ | ||
| </h3> | </h3> | ||
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