---
title: "A cosmic ray proton with mass \\(m\\) and charge \\(e\\) travels vertically downward toward Earth’s surface along the equatorial plane at speed \\(v\\). Near the equator, Earth’s magnetic field is horizontal, directed from South to North, with uniform magnitude \\(B\\). Which of the following correctly identifies the direction of the initial magnetic force acting on the proton and the radius of curvature \\(R\\) of its trajectory?"
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url: "https://nerd-notes.com/ubq/118576/"
date_modified: "2026-08-04T08:11:24+00:00"
---

# A cosmic ray proton with mass \(m\) and charge \(e\) travels vertically downward toward Earth’s surface along the equatorial plane at speed \(v\). Near the equator, Earth’s magnetic field is horizontal, directed from South to North, with uniform magnitude \(B\). Which of the following correctly identifies the direction of the initial magnetic force acting on the proton and the radius of curvature \(R\) of its trajectory?

A cosmic ray proton with mass \(m\) and charge \(e\) travels vertically downward toward Earth's surface along the equatorial plane at speed \(v\). Near the equator, Earth's magnetic field is horizontal, directed from South to North, with uniform magnitude \(B\). Which of the following correctly identifies the direction of the initial magnetic force acting on the proton and the radius of curvature \(R\) of its trajectory?

![A schematic diagram showing a cross-section near Earth's equator. Earth's surface is represented by a horizontal line at the bottom. A proton marked with a plus sign moves toward the surface along a downward arrow labeled v. A horizontal arrow parallel to the surface pointing to the right is labeled B, pointing North. A directional key in the top right corner indicates North pointing right, East pointing into the page, and Down pointing toward the bottom of the diagram. No other labels, lines, text, or axes appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1785831084-rIX7EZ.jpg)

- **A.** Directed toward the East, with radius \(R = \dfrac{mv}{eB}\)
- **B.** Directed toward the West, with radius \(R = \dfrac{mv}{eB}\)
- **C.** Directed toward the East, with radius \(R = \dfrac{eB}{mv}\)
- **D.** Directed toward the West, with radius \(R = \dfrac{eB}{mv}\)

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/118576/*