Class 12 physics chapterwise important questions for board exam

Check out the important conceptual questions of rotational dynamics.

1. A man standing on the turn table extends his hands suddenly. What happens?
   Ans: By extending the hand, the moment of inertia of the system increases and thus angular velocity ω decreases. Now, the turn table will slow down as Iω remains constant in the absence of any external torque.
2. Explain why spokes are fitted in the cycle wheel.
   Ans: The spokes of cycle wheel increases its moment of inertia. The greater the moment of inertia, the more is the opposition to any change in uniform rotational motion. As a result, cycle runs smoothly and steadily. If the cycle wheels have no spokes, the cycle will be driven jerky and hence be unsafe.
3. If the earth is strucked by meteorites, the earth will slow down slightly. Why?
   Ans: When the earth is strucked by meteorites, it remains on the earth and the moment of inertia of the earth increases. As a result, the angular velocity decreases. Hence, the earth slow down slightly.
4. Why there are two propellers in a helicopter?
   Ans: if there were only one propeller in the helicopter, the helicopter itself would have turned in opposite direction due to conservation of angular momentum. Hence, there are two propellers in helicopter to balance it.
5. The angular velocity of the earth around the sun increases when it comes closer to the sun. Why?
   Ans: When the earth come closer to the sun, it's moment of inertia about the axis through the sun decreases. To conserve angular momentum (L = Iω), the angular velocity of the earth increases.
6. If a body is rotating, is it necessarily being acted upon by an external torque?
   Ans: Torque is required only for producing angular acceleration. For uniform rotation, no torque is needed.
7. Does the angular momentum of a body moving in a circular path change? Give explanation to your answers.
   Ans: No, the angular momentum of a body moving in a circular path doesn't change because the angular momentum always remains conserved if the external torque on it is zero. i.e. L = Iω remain constant where I is moment of inertia and ω is angular speed.
8. Why a wrench of longer arm is preferred in comparison to a wrench of shorter arm?
   Ans: A torque is applied to move a wrench, where torque = force × perpendicular distance from the axis of rotation. A smaller force can be applied if the wrench has longer arm for the same torque. We feel easier with small force and therefore, we prefer a wrench of longer arm as compared to that of shorter arm.