---
title: "A student applies a constant horizontal force \\(F_P\\) to Block 1 of mass \\(m\\). Block 1 is in contact with Block 2 of mass \\(3m\\), and both blocks accelerate together to the right across a horizontal surface where friction is negligible. Which of the following correctly compares the magnitude of the force Block 1 exerts on Block 2, \\(F_{12}\\), to the magnitude of the force Block 2 exerts on Block 1, \\(F_{21}\\)?"
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url: "https://nerd-notes.com/ubq/109261/"
date_modified: "2026-03-21T18:52:14+00:00"
---

# A student applies a constant horizontal force \(F_P\) to Block 1 of mass \(m\). Block 1 is in contact with Block 2 of mass \(3m\), and both blocks accelerate together to the right across a horizontal surface where friction is negligible. Which of the following correctly compares the magnitude of the force Block 1 exerts on Block 2, \(F_{12}\), to the magnitude of the force Block 2 exerts on Block 1, \(F_{21}\)?

A student applies a constant horizontal force \(F_P\) to Block 1 of mass \(m\). Block 1 is in contact with Block 2 of mass \(3m\), and both blocks accelerate together to the right across a horizontal surface where friction is negligible. Which of the following correctly compares the magnitude of the force Block 1 exerts on Block 2, \(F_{12}\), to the magnitude of the force Block 2 exerts on Block 1, \(F_{21}\)?

![A horizontal surface with two blocks in contact. Block 1, a smaller rectangle labeled 'm', is on the left. Block 2, a larger rectangle labeled '3m', is on the right. A horizontal arrow labeled 'Fp' points to the right and is applied to the left side of Block 1.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1774119134-bungwL.jpg)

- **A.** \(F_{12} > F_{21}\) because Block 1 must provide a net force sufficient to overcome the larger inertia of Block 2.
- **B.** \(F_{12} = F_{21}\) because the forces are members of an action-reaction pair and must be equal regardless of the masses or acceleration.
- **C.** \(F_{21} > F_{12}\) because Block 2 has three times the mass of Block 1 and therefore exerts a greater reaction force.
- **D.** \(F_{12} = F_{21}\) only if the blocks move at a constant velocity, but \(F_{12} > F_{21}\) while the system is accelerating.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/109261/*