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title: "A man with mass \\( m \\) is standing on a rotating platform in a science museum. The platform can be approximated as a uniform disk of radius \\( R \\) that rotates without friction at a constant angular velocity \\( \\omega \\). The surface of the platform is frictionless, so the only forces between the man and the platform arise from the man’s feet as he runs. Two students are discussing what the man should do if he wishes to remain directly above a single point on the platform’s surface (so that, as viewed from the ground, he does not drift relative to that point).Student A claims that the man should run clockwise, in the same direction that the platform is rotating, because doing so will decrease the system’s moment of inertia and therefore increase \\( \\omega \\), allowing him to stay above the desired point.Student B claims that, because no external torque acts on the man–platform system, the man must instead run counter-clockwise (opposite the platform’s rotation) so that the total angular momentum of the system about its central axis is conserved.Briefly explain which student’s reasoning is correct, explicitly referring to conservation of angular momentum and the absence (or presence) of external torques."
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url: "https://nerd-notes.com/ubq/91125/"
date_modified: "2025-11-03T08:44:46+00:00"
---

# A man with mass \( m \) is standing on a rotating platform in a science museum. The platform can be approximated as a uniform disk of radius \( R \) that rotates without friction at a constant angular velocity \( \omega \). The surface of the platform is frictionless, so the only forces between the man and the platform arise from the man’s feet as he runs. Two students are discussing what the man should do if he wishes to remain directly above a single point on the platform’s surface (so that, as viewed from the ground, he does not drift relative to that point).Student A claims that the man should run clockwise, in the same direction that the platform is rotating, because doing so will decrease the system’s moment of inertia and therefore increase \( \omega \), allowing him to stay above the desired point.Student B claims that, because no external torque acts on the man–platform system, the man must instead run counter-clockwise (opposite the platform’s rotation) so that the total angular momentum of the system about its central axis is conserved.Briefly explain which student’s reasoning is correct, explicitly referring to conservation of angular momentum and the absence (or presence) of external torques.

A man with mass \( m \) is standing on a rotating platform in a science museum. The platform can be approximated as a uniform disk of radius \( R \) that rotates without friction at a constant angular velocity \( \omega \). Two students are discussing what the man should do if he wishes to change the angular velocity of the platform.

Student A says that the man should run towards the center of the platform, because this will decrease the moment of inertia of the man-platform system. Since \( L \propto I \), the angular momentum will decrease proportionately and the platform will slow down.

Student B says that since the platform is rotating counterclockwise, the man should run in a clockwise direction to slow the platform down. His feet will exert a frictional torque on the platform, which will cause an angular acceleration of the man-platform system.

Explain what is correct and incorrect about each students statement if anything.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/91125/*