---
title: "A block of mass \\(m\\) and a block of mass \\(3m\\) are connected by an ideal string passing over a frictionless pulley of negligible mass. Initially, the block of mass \\(3m\\) is held in place by a student so that the system is at rest. The student then releases the block, allowing the system to accelerate. How does the tension in the string change after the block of mass \\(3m\\) is released?"
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url: "https://nerd-notes.com/ubq/109259/"
date_modified: "2026-03-21T18:52:13+00:00"
---

# A block of mass \(m\) and a block of mass \(3m\) are connected by an ideal string passing over a frictionless pulley of negligible mass. Initially, the block of mass \(3m\) is held in place by a student so that the system is at rest. The student then releases the block, allowing the system to accelerate. How does the tension in the string change after the block of mass \(3m\) is released?

A block of mass \(m\) and a block of mass \(3m\) are connected by an ideal string passing over a frictionless pulley of negligible mass. Initially, the block of mass \(3m\) is held in place by a student so that the system is at rest. The student then releases the block, allowing the system to accelerate. How does the tension in the string change after the block of mass \(3m\) is released?

![An Atwood machine with a pulley fixed to a ceiling. A string passes over the pulley. On the left side, a small block is labeled m. On the right side, a larger block is labeled 3m. A hand is shown holding the 3m block to keep it stationary.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1774119133-RDexKQ.jpg)

- **A.** The tension increases because the block of mass \(m\) must have a net upward force acting on it to accelerate upward.
- **B.** The tension decreases because the block of mass \(3m\) is accelerating downward, which reduces the force it exerts on the string.
- **C.** The tension stays the same because the total mass of the system is unchanged and the pulley is ideal.
- **D.** The tension stays the same because the tension in the string of an ideal Atwood machine is always equal to the weight of the lighter mass.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/109259/*