---
title: "Two blocks of mass \\(M\\) and \\(3M\\) are placed on a horizontal, frictionless surface with an ideal spring of spring constant \\(k\\) compressed by a distance \\(d\\) between them. The blocks are not attached to the spring, and the system is released from rest. At the moment the spring reaches its uncompressed length and separates from the blocks, what is the kinetic energy of the block of mass \\(M\\)?"
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url: "https://nerd-notes.com/ubq/124004/"
date_modified: "2026-09-28T13:30:09+00:00"
---

# Two blocks of mass \(M\) and \(3M\) are placed on a horizontal, frictionless surface with an ideal spring of spring constant \(k\) compressed by a distance \(d\) between them. The blocks are not attached to the spring, and the system is released from rest. At the moment the spring reaches its uncompressed length and separates from the blocks, what is the kinetic energy of the block of mass \(M\)?

Two blocks of mass \(M\) and \(3M\) are placed on a horizontal, frictionless surface with an ideal spring of spring constant \(k\) compressed by a distance \(d\) between them. The blocks are not attached to the spring, and the system is released from rest. At the moment the spring reaches its uncompressed length and separates from the blocks, what is the kinetic energy of the block of mass \(M\)?

![A side-view schematic of a horizontal mechanical system. A single horizontal solid baseline spans the bottom to represent a frictionless floor. Two rectangular blocks rest on this baseline. On the left is a smaller rectangle with label \(M\) centered inside it. On the right is a larger rectangle with label \(3M\) centered inside it. Positioned horizontally between the inner vertical faces of the two blocks is an ideal helical spring shown in compression, consisting of exactly six zigzag coils and labeled \(k\) above its center. A horizontal dimension line with arrows at both ends spans the compressed spring directly beneath it, labeled \(d\). No other labels, lines, text, or axes appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1790602209-H70rNt.jpg)

- **A.** \(\dfrac{1}{8}kd^2\)
- **B.** \(\dfrac{1}{4}kd^2\)
- **C.** \(\dfrac{3}{8}kd^2\)
- **D.** \(\dfrac{1}{2}kd^2\)

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/124004/*