Правка разделов «Statement», «Solution», «Answer»

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### Statement
−$3.3.31.$ [Insert the problem statement]
+$3.3.31.$ To measure small amplitudes of oscillations of a diaphragm making harmonic vibrations of high frequency $\omega$, a ”hammer” connected in an electrical circuit with a diaphragm and a telephone is used. The hammer of mass $m$ is pressed against the diaphragm with a force that is adjusted by a micrometer screw. When the contact of the hammer with the diaphragm is interrupted, the current in the circuit is interrupted and a rattling can be heard in the telephone. Determine the amplitude of the vibrations if the rattling begins when the force with which the hammer presses the diaphragm reaches the value $F$.
+![For problem $3.3.31$|464x817, 50%](../../img/3.3.31/3.3.31.png)
+
+
### Solution
−213
+This promlem is analogous to the previous one ([$3.3.30$](/en/3.3.30); if anything, look at the solution of the mentioned problem in russian). We can write for the hammer (projection onto the axis of oscillations)
+$$
+F-N=ma,
+$$
+
+where $N$ is the normal reaction force of the diaphragm, $a$ is the acceleration of the hammer.
+
+The diaphragm perform harmonic oscillations, thus
+
+$$
+a=\omega^2A\sin\omega t,
+$$
+
+where $A$ is the oscillation amplitude of the diaphragm, $t$ is time period.
+
+From the previous equations we can obtain
+
+$$
+N=F-m\omega^2A\sin\omega t.
+$$
+
+If the value of $F$ is larger than $m\omega^2A$, then $N>0$ and there will be no the rattling (since $-1\leqslant\sin\omega t\leqslant1$). The rattling begins, when
+
+$$
+F=m\omega^2A
+$$
+
+and therefore
+
+$$
+A=\frac{F}{m\omega^2}.
+$$
+
+
#### Answer
−[Insert a concise answer or boxed result]
+$$
+A=\frac{F}{m\omega^2}.
+$$