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Rotational Mechanics — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsRotational Mechanics1 Mark
Question
A particle moves on a straight line with a uniform velocity. Its angular momentum:
  1. Is always zero.
  2. Is zero about a point on the straight line.
  3. Is not zero about a point away from the straight line.
  4. About any given point remains constant.
✓

Answer

  1. Is zero about a point on the straight line.
  2. Is not zero about a point away from the straight line.
  3. About any given point remains constant.

Explanation:

  1. Angular momentum $=\text{m}\Big(\overrightarrow{\text{r}}\times\overrightarrow{\text{v}}\Big)$

If the point is on the straight line, $\overrightarrow{\text{r}}$ and $\overrightarrow{\text{v}}$ will have the same direction and their cross product will be zero. Hence, angular momentum is zero.

  1. If the point is not on the straight line, $\overrightarrow{\text{r}}$ and $\overrightarrow{\text{v}}$ will not have the same direction and their cross product will not be zero. Hence, angular momentum is non-zero.
  2. No external torque is applied on the body; therefore, its angular momentum about any given point remains constant.

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