Class 12 physics chapterwise important questions for board exam
Check out the important conceptual questions of rotational dynamics.
Rotational dynamics
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.