Q221 / 240 · KCET 2026 · Physics
The angular momentum of a moving body remains constant, if
- Anet external force is applied
- Bnet pressure is applied
- Cnet external torque is applied
- Correct answerDnet external torque is not applied
Solution
According to Newton's second law for rotational motion, the rate of change of angular momentum of a body is equal to the net external torque acting on it.
$\dfrac{d\vec{L}}{dt} = \vec{\tau}_{ext}$
If the net external torque applied on the body is zero ($\vec{\tau}_{ext} = 0$), then $\dfrac{d\vec{L}}{dt} = 0$.
This implies that $\vec{L} = \text{constant}$.
Thus, the angular momentum of a moving body remains constant if net external torque is not applied.
Answer: net external torque is not applied
$\dfrac{d\vec{L}}{dt} = \vec{\tau}_{ext}$
If the net external torque applied on the body is zero ($\vec{\tau}_{ext} = 0$), then $\dfrac{d\vec{L}}{dt} = 0$.
This implies that $\vec{L} = \text{constant}$.
Thus, the angular momentum of a moving body remains constant if net external torque is not applied.
Answer: net external torque is not applied
Hence, the correct answer is option: 'D'.