---
title: "Tension — AP Physics C Mechanics Definition & Exam Guide"
description: "Tension is the pulling force a string or cable transmits along its length. Learn how it drives Atwood machines, pendulums, and circular motion on AP Physics C."
canonical: "https://fiveable.me/ap-physics-c-mechanics/key-terms/tension"
type: "key-term"
subject: "AP Physics C: Mechanics"
unit: "Unit 2"
---

# Tension — AP Physics C Mechanics Definition & Exam Guide

## Definition

Tension is the macroscopic pulling force that segments of a string, cable, or chain exert on each other (and on attached objects) in response to an external force. In an ideal massless string, tension is the same everywhere along its length and always pulls along the string, never pushes.

## What It Is

Tension is what happens when you zoom out on a string under load. Every tiny segment of the string pulls on its neighbors, and the net macroscopic result is a [force](/ap-physics-c-mechanics/unit-2/2-forces-and-free-body-diagrams/study-guide/2LH73zRqxtRXtAKH "fv-autolink") transmitted along the string's length. That's why tension always points along the string and always pulls. A rope can't push anything.

On [AP Physics C](/ap-physics-c-mechanics "fv-autolink"), you'll almost always work with the **ideal string**, which is massless and doesn't stretch. Those two assumptions buy you a lot. Massless means the tension is the same magnitude at every point along the string (even over an [ideal pulley](/ap-physics-c-mechanics/key-terms/ideal-pulley "fv-autolink")), and inextensible means objects tied together share the same speed and acceleration magnitude. Tension is a microscopic Newton's third law story (segment pulls segment, segment pulls back) packaged into one clean variable T you can drop into a free-body diagram.

## Why It Matters

Tension lives in [Unit 2](/ap-physics-c-mechanics/unit-2 "fv-autolink") (Force and Translational Dynamics), showing up directly in Topic 2.3 (Newton's Third Law) and Topic 2.10 (Circular Motion). It's one of the most common forces in free-body diagrams across the whole course. In Topic 2.3, tension is the textbook example of third-law pairs at work, since the string pulls on the block and the block pulls right back on the string. In Topic 2.10, tension is frequently the force providing (or contributing to) the [centripetal acceleration](/ap-physics-c-mechanics/key-terms/centripetal-acceleration "fv-autolink"), as in conical pendulums and vertical circles. If you can't handle tension cleanly, you can't solve Atwood machines, pendulums, banked-curve-style circular motion, or pulley-and-incline systems, and those are FRQ staples.

## Connections

### [Equal and opposite forces (Unit 2)](/ap-physics-c-mechanics/key-terms/equal-and-opposite-forces)

Tension is [Newton's third law](/ap-physics-c-mechanics/unit-2/3-newtons-third-law/study-guide/SXl4nBHlUrotvxSj "fv-autolink") made visible. When a string pulls a block with force T, the block pulls the string back with force T. That microscopic tug-of-war between adjacent string segments is literally what tension is.

### [Ideal pulley (Unit 2)](/ap-physics-c-mechanics/key-terms/ideal-pulley)

A frictionless, massless pulley redirects a string without changing its tension. That's the whole trick behind the Atwood machine, where one tension value T connects two separate free-body diagrams. Once the pulley has mass ([Unit 5](/ap-physics-c-mechanics/unit-5 "fv-autolink")), the tensions on each side are no longer equal.

### [Conical pendulum (Unit 2)](/ap-physics-c-mechanics/key-terms/conical-pendulum)

In a [conical pendulum](/ap-physics-c-mechanics/key-terms/conical-pendulum "fv-autolink"), tension does two jobs at once. Its vertical component balances gravity (T cos θ = mg) while its horizontal component supplies the centripetal force (T sin θ = mv²/r). Resolving one tension vector into two roles is a classic exam move.

### [Internal forces (Unit 4)](/ap-physics-c-mechanics/key-terms/internal-forces)

When you treat two connected blocks as a single system, the string tension becomes an internal force and cancels out of the system's equation of motion. Choosing whether tension is internal or external is really just choosing your system boundary.

## On the AP Exam

Tension is everywhere on this exam. The 2017 FRQs alone used it three times, in an Atwood machine (Q1), a block on an incline (Q2), and a rolling cylinder setup (Q3), and the 2019 exam used it again in a pendulum collision problem (Q2). The standard task is to draw a correct free-body diagram with T pointing along the string, write Newton's second law for each object, and solve the system of equations. In circular motion problems, expect to resolve tension into components (conical pendulum) or evaluate it at specific points on a vertical circle. A favorite MCQ asks where tension is minimum in a vertical circle. It's at the top, where gravity helps supply the centripetal force, giving T = mv²/r − mg. Common point-losers are drawing tension pushing instead of pulling, assuming tension equals the hanging weight even when the system accelerates, and forgetting that tension changes once a pulley has mass.

## tension vs Centripetal force

Tension is an actual force exerted by a string. Centripetal force is not a separate force at all; it's the name for whatever net force component points toward the center of a circular path. In a conical pendulum, the horizontal component of tension IS the centripetal force. Never draw a separate 'centripetal force' arrow on a free-body diagram. The College Board will dock you for it. Draw the real forces (tension, gravity, normal) and let their net result point toward the center.

## Key Takeaways

- Tension always pulls along the direction of the string and can never push, because a rope only resists being stretched.
- In an ideal (massless, inextensible) string, the tension has the same magnitude at every point, even when the string passes over a frictionless, massless pulley.
- Tension in an accelerating system does not equal the hanging weight; in an Atwood machine, T is between m₁g and m₂g, never equal to either.
- In a conical pendulum, the vertical component of tension balances gravity while the horizontal component provides the centripetal acceleration.
- For an object moving in a vertical circle at constant speed, tension is minimum at the top of the circle, where T = mv²/r − mg, and maximum at the bottom.
- Never label 'centripetal force' as its own arrow on a free-body diagram; tension or another real force plays that role.

## FAQs

### What is tension in AP Physics C Mechanics?

Tension is the macroscopic pulling force that segments of a string, cable, or chain exert on each other in response to an external force. It transmits force along the string, always pulls (never pushes), and in an ideal massless string it has the same magnitude everywhere.

### Is tension the same as centripetal force?

No. Centripetal force is a role, not a real force. Tension is a real force that can play the centripetal role, like the horizontal component of string tension in a conical pendulum. On free-body diagrams, draw tension, not a separate centripetal arrow.

### Is tension always equal to the weight of the hanging object?

No, only when the object is in equilibrium. If the system accelerates, like in an Atwood machine, the tension falls between the two weights. The 2017 FRQ Q1 tested exactly this with two blocks over a frictionless pulley.

### Where is tension minimum in a vertical circle?

At the top of the circle. There, gravity already points toward the center and helps supply the centripetal force, so the string only needs T = mv²/r − mg. At the bottom, tension is maximum at T = mv²/r + mg.

### Is tension the same on both sides of a pulley?

Only if the pulley is ideal (massless and frictionless), which is the standard Unit 2 assumption. Once the pulley has rotational inertia, in Unit 5, the tensions must differ to produce a net torque that spins the pulley.

## Related Study Guides

- [2.10 Circular Motion](/ap-physics-c-mechanics/unit-2/10-circular-motion/study-guide/mSTvL7QY6udY9crx)

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