Added 1.2.20-1.2.21; 1.3.1-1.3.2

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+ <meta name="description" content="Two balls with velocity v are thrown vertically upwards from the same point with time interval \Delta t. How long after the second ball leaves, will they collide?">
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+ <h2>Solutions of Savchenko Problems in Physics</h2>
+ <p class="author">
+ Aliaksandr Melnichenka <br/>
+ October 2023
+ </p>
+ </header>
+
+ <h3 id="back-link"><a href="../#1.3">$\leftarrow$Back</a></h3>
+
+ <h3> Statement </h3>
+ <p>
+ $1.3.1.$ Two balls with velocity $v$ are thrown vertically upwards from the same point with time interval $\Delta t$. How long after the second ball leaves, will they collide?
+ </p>
+
+ <h3>Solution</h3>
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+The velocity of the first ball after an interval of time $\Delta t$:
+$$v_0 = v - g \Delta t$$
+The relative velocity of the 1st and 2nd balls will remain constant and equal:
+$$v_\text{average} = g \Delta t$$
+Meanwhile, during the time $\Delta t$, the 1st ball has risen to a height of
+$$L = v_0 \Delta t - \frac{g \Delta t^2}{2}$$
+The time in which they will collide will find how:
+$$t = \frac{L}{v_\text{average}}$$
+$$\fbox{$t = \frac{v}{g} - \frac{\Delta t}{2}$}$$
+
+ </p>
+
+ <h4>Answer</h4>
+ <p>
+ $$t = v/g − \Delta t/2$$
+ </p>
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