---
title: "A platform is initially rotating on smooth ice with negligible friction, as shown at left in the figure. A stationary disk is dropped directly onto the center of the platform. A short time later, the disk and platform rotate together at the same angular velocity, as shown at right in the figure. How does the angular momentum of only the platform change, if at all, after the disk drops? What is a justification for your answer?"
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url: "https://nerd-notes.com/ubq/28504/"
date_modified: "2025-03-11T04:57:57+00:00"
---

# A platform is initially rotating on smooth ice with negligible friction, as shown at left in the figure. A stationary disk is dropped directly onto the center of the platform. A short time later, the disk and platform rotate together at the same angular velocity, as shown at right in the figure. How does the angular momentum of only the platform change, if at all, after the disk drops? What is a justification for your answer?

A platform is initially rotating on smooth ice with negligible friction, as shown above. A stationary disk is dropped directly onto the center of the platform. A short time later, the disk and platform rotate together at the same angular velocity, as shown at right in the figure. How does the angular momentum of only the platform change, if at all, after the disk drops? And what is the best justification.

![Diagram](https://nerd-notes.com/wp-content/uploads/2024/03/Screen-Shot-2024-03-23-at-7.12.08-PM-300x101.png)

- **A.** It decreases. The top disk exerts a torque on the platform.
- **B.** It decreases. The potential energy of the platform-disk-Earth system decreases when the disk drops.
- **C.** It stays the same. Angular momentum is a conserved quantity.
- **D.** It stays the same. The torques that the disk and platform exert on each other are equal in magnitude but in opposite directions.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/28504/*