Question
Derive the expression for resultant spring constant when two springs having constants $k_1$ and $k_2$ are connected
  1. In parallel.
  2. In series.

Answer

  1. When the springs are connected in parallel, the extension in them will be same and the total restoring force is the sum of their restoring forces.
$\therefore\text{F = F}_1+\text{F}_2$
$-\text{k}_{\text{eq}}\text{x}=-\text{k}_1\text{x}-\text{k}_2\text{x}$
$\text{k}_{\text{eq}}=\text{k}_1+\text{k}_2$
  1. When the springs are connected in series, the restoring force is same in both the springs and the extensions will be different so the not extension
i.e., $\text{x = x}_1+\text{x}_2$
$=\frac{\text{F}}{-\text{k}_{\text{eq}}}=\frac{-\text{F}}{\text{k}_1}-\frac{\text{F}}{\text{k}_2}$
$\therefore\frac{1}{\text{k}_{\text{eq}}}=\frac{1}{\text{k}_1}+\frac{1}{\text{k}_2}$
when connected in series.

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