A particle moves along a straight line according to the law $s=\frac{1}{3} t^3-3 t^2+9 t+17$, where $s$ is…

A particle moves along a straight line according to the law $s=\frac{1}{3} t^3-3 t^2+9 t+17$, where $s$ is in metre and $t$ is in second. Its velocity decreases in
  1. $0 < t < 5$
  2. $0 < t < 3$
  3. $t>5$
  4. $t>3$

Solution

We have, $ \begin{aligned} & S=\frac{1}{3} t^3-3 t^2+9 t+17 \\ & V=\frac{d s}{d t}=t^2-6 t+9 \\ & V=t^2-6 t+9 \\ & \frac{d V}{d t}=2 t-6 \end{aligned} $ Velocity is decreasing, if $\frac{d V}{d t} < 0$ $ \begin{array}{ll} \therefore \quad & 2 t-6 < 0 \\ & 0 < t < 3$ [\because t>0] \end{array} $

Asked in: AP EAMCET 2021 (25 Aug Shift 1)

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