---
title: "A system consists of Block A of mass \\(m\\), Block B of mass \\(2m\\), and Earth. The blocks are connected by a light string that passes over a frictionless pulley of negligible mass. Block B is held at a height \\(h\\) above the floor while Block A rests on the floor. The system is released from rest and Block B accelerates toward the floor. Which of the following correctly describes the change in the total gravitational potential energy of the system from the moment of release until the moment Block B hits the floor?"
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url: "https://nerd-notes.com/ubq/113119/"
date_modified: "2026-05-05T04:25:37+00:00"
---

# A system consists of Block A of mass \(m\), Block B of mass \(2m\), and Earth. The blocks are connected by a light string that passes over a frictionless pulley of negligible mass. Block B is held at a height \(h\) above the floor while Block A rests on the floor. The system is released from rest and Block B accelerates toward the floor. Which of the following correctly describes the change in the total gravitational potential energy of the system from the moment of release until the moment Block B hits the floor?

A system consists of Block A of mass \(m\), Block B of mass \(2m\), and Earth. The blocks are connected by a light string that passes over a frictionless pulley of negligible mass. Block B is held at a height \(h\) above the floor while Block A rests on the floor. The system is released from rest and Block B accelerates toward the floor. Which of the following correctly describes the change in the total gravitational potential energy of the system from the moment of release until the moment Block B hits the floor?

![An Atwood machine setup. A small pulley is fixed at the top. Block A, labeled m, is on the floor on the left side of the pulley. Block B, labeled 2m, is hanging on the right side of the pulley at a vertical height h above the floor. A string connects the two blocks over the pulley.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1777955136-hR0Vok.jpg)

- **A.** The gravitational potential energy decreases by \(2mgh\) because Block B is the only object in the system that loses height during the motion.
- **B.** The gravitational potential energy decreases by \(mgh\) because the increase in potential energy of Block A partially offsets the decrease in potential energy of Block B.
- **C.** The gravitational potential energy stays the same because the total mechanical energy of the Block A-Block B-Earth system is conserved.
- **D.** The gravitational potential energy increases by \(mgh\) because the work done by the string on Block A is positive, leading to a gain in the system's potential energy.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/113119/*