---
title: "A solid copper plate attached to a light rod swings as a pendulum, entering a region of uniform magnetic field directed perpendicular to the plane of the plate’s motion. As the plate enters the magnetic field region, it rapidly decelerates and comes to rest much faster than it would in air alone. Which of the following best explains the physical mechanism responsible for this magnetic braking effect?"
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url: "https://nerd-notes.com/ubq/118635/"
date_modified: "2026-08-04T08:13:23+00:00"
---

# A solid copper plate attached to a light rod swings as a pendulum, entering a region of uniform magnetic field directed perpendicular to the plane of the plate’s motion. As the plate enters the magnetic field region, it rapidly decelerates and comes to rest much faster than it would in air alone. Which of the following best explains the physical mechanism responsible for this magnetic braking effect?

A solid copper plate attached to a light rod swings as a pendulum, entering a region of uniform magnetic field directed perpendicular to the plane of the plate's motion. As the plate enters the magnetic field region, it rapidly decelerates and comes to rest much faster than it would in air alone. Which of the following best explains the physical mechanism responsible for this magnetic braking effect?

![A metal pendulum swinging toward a region of magnetic field. At the top, a pivot point holds a vertical thin rod connected to a flat, solid copper rectangular plate at the bottom. Below the pivot, a defined rectangular region represents a uniform magnetic field directed into the page, filled with a grid of x symbols. The pendulum plate is positioned at the left boundary of this magnetic field region, moving horizontally to the right as indicated by a rightward velocity arrow labeled v. No other labels or lines appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1785831202-Cf5kWn.jpg)

- **A.** As the plate enters the magnetic field region, the changing magnetic flux induces circulating eddy currents whose magnetic force opposes the motion of the plate in accordance with Lenz's law.
- **B.** The uniform magnetic field exerts a direct magnetic force on the stationary, neutral atoms of the conductor, pulling the entire plate toward the center of the field region.
- **C.** As the plate enters the field region, induced eddy currents generate an internal magnetic field aligned with the external field, attracting the plate further into the field.
- **D.** Eddy currents are only induced when the magnetic field magnitude changes over time, so the observed deceleration is entirely caused by air drag enhanced by the presence of the field.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/118635/*