A particle moves on a straight line with a uniform velocity. Its angular momentum __________ .

(a) is always zero

(b) is zero about a point on the straight line

(c) is not zero about a point away from the straight line

(d) about any given point remains constant.

#### Solution

(b) is zero about a point on the straight line

(c) is not zero about a point away from the straight line

(d) about any given point remains constant

(b) Angular momentum = \[m\left( \overrightarrow{r} \times \overrightarrow{v} \right)\]

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

(c) If the point is not on the straight line, \[\overrightarrow{r} \text{ and } \overrightarrow{v}\] will not have the same direction and their cross product will not be zero. Hence, angular momentum is non-zero.

(d) No external torque is applied on the body; therefore, its angular momentum about any given point remains constant.