Question:

A particle of mass $ m $ is moving with a constant velocity along a line parallel to the positive direction of $ X $ -axis. The magnitude of its angular momentum with respect to the origin

Updated On: Jun 14, 2022
  • is zero
  • goes on increasing as $ x $ increases
  • goes on decreasing as $ x $ increases
  • remains constant for all positions of the particle
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The Correct Option is D

Solution and Explanation

According to the question,
Given, mass of the particle $= m$

$\therefore$ Linear momentum of the particle, $p=mV$
or $=mV \hat{i}$
Position of particle at time $t$, $r=x\hat{i}+y \hat{J} $
$=Vt \hat{i}+y \hat{J} \left(\because x=Vt\right)$
$\therefore$ Angular momentum of the particle about $O$
$L=r \times p=\left[Vt \hat{i}+y\hat{J} \right]\times mV \hat{i} $
$L=\left|\begin{matrix}\hat{i}&\hat{j}&\hat{k}\\ vt&y&0\\ v&0&0\end{matrix}\right|$
$L=\hat{i}\left(0-0\right)-\hat{j} \left(0-0\right)+\hat{k} \left(0-vy\right)$
$=-Vy\hat{k}$ (constant)
$(\because$ Particle is moving in +ve $x$-direction )
Hence, angular momentvun of particle w.r.t the origin remains constant for all positions of the particle
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Concepts Used:

System of Particles and Rotational Motion

  1. The system of particles refers to the extended body which is considered a rigid body most of the time for simple or easy understanding. A rigid body is a body with a perfectly definite and unchangeable shape.
  2. The distance between the pair of particles in such a body does not replace or alter. Rotational motion can be described as the motion of a rigid body originates in such a manner that all of its particles move in a circle about an axis with a common angular velocity.
  3. The few common examples of rotational motion are the motion of the blade of a windmill and periodic motion.