---
title: "External Forces — AP Physics 1 Definition & Exam Guide"
description: "External forces come from outside your defined system and can change its total momentum or energy. Defining the system correctly is the key AP Physics 1 skill."
canonical: "https://fiveable.me/ap-physics-1-revised/key-terms/external-forces"
type: "key-term"
subject: "AP Physics 1"
unit: "Unit 5"
---

# External Forces — AP Physics 1 Definition & Exam Guide

## Definition

In AP Physics 1, external forces are forces exerted on a system by objects outside that system. They can change the system's total momentum, energy, or angular momentum, while internal forces (between objects inside the system) cannot change the system's totals.

## What It Is

An external [force](/ap-physics-1-revised/unit-2/2-forces-and-free-body-diagrams/study-guide/jQ2Obd0dAU4QiTPN "fv-autolink") is any force acting on your system that comes from something *outside* the system. The catch is that "external" isn't a property of the force itself. It depends entirely on how you draw the boundary of your system. Friction from the floor is external if your system is just the block, but if you include the floor in your system, that same friction becomes internal.

This matters because of one big rule. Only external forces can change a system's total momentum, total angular momentum, or [total mechanical energy](/ap-physics-1-revised/key-terms/total-mechanical-energy "fv-autolink"). Internal forces always come in Newton's third law pairs that cancel out within the system. So when a problem says "frictionless surface" or "ignore air resistance," it's really telling you the net external force (or external work) is zero, which means a conservation law applies. Defining your system so that messy forces become internal is one of the most powerful moves in [AP Physics 1](/ap-physics-1-revised "fv-autolink").

## Why It Matters

External forces show up everywhere [conservation laws](/ap-physics-1-revised/unit-6/6-motion-of-orbiting-satellites/study-guide/tzB1DCcspZo8vXIC "fv-autolink") do. In Topic 5.2 (Representations of Changes in Momentum), whether momentum is conserved comes down to one question. Is the net external force on the system zero? In a [collision](/ap-physics-1-revised/key-terms/collision "fv-autolink") between two blocks, the forces they exert on each other are internal, so the system's total momentum stays constant even though each block's individual momentum changes. In Topic 6.1, external forces like gravity and spring forces drive simple harmonic oscillators and supply the restoring force that pulls the object back toward equilibrium. The system-definition skill behind this term also threads through Units 5 and 6 in the rotational world, since only external torques can change a rigid system's angular momentum or rotational kinetic energy (LO 6.1.A). On the exam, the very first thing you should do in almost any momentum or energy problem is decide what's in your system and which forces are external to it.

## Connections

### [Internal Forces (Units 4-5)](/ap-physics-1-revised/key-terms/internal-forces)

These are two sides of the same boundary. [Internal forces](/ap-physics-1-revised/key-terms/internal-forces "fv-autolink") act between objects inside your system and always cancel in third-law pairs, so they can never change the system's total momentum. External forces are the only ones that can. The same physical force flips between internal and external depending on where you draw the system.

### [Net Force (Units 1-2)](/ap-physics-1-revised/key-terms/net-force)

[Newton's second law](/ap-physics-1-revised/unit-2/5-newtons-second-law/study-guide/FizcgPbKTypwBNrG "fv-autolink") for a system uses the net *external* force, not the sum of every force in sight. When you write F_net = ma for a multi-object system, the internal forces have already canceled, and only external pushes and pulls move the center of mass.

### Frictional Force (Units 2 and 5)

Friction is the classic external force that ruins conservation. Friction from a surface outside your system removes [mechanical energy](/ap-physics-1-revised/key-terms/mechanical-energy "fv-autolink") and changes momentum, which is why exam problems carefully tell you whether surfaces are frictionless before expecting you to conserve anything.

### Equilibrium Position (Topic 6.1)

A simple harmonic oscillator sits at equilibrium when the external forces on it balance. Displace it, and an external restoring force (the spring or gravity) pulls it back, which is what creates oscillation in the first place.

## On the AP Exam

The exam rarely asks "define external force." Instead, it tests whether you can use the idea to justify a conservation claim. The 2024 Short FRQ Q5 is the template. Block A slides into Block B, and the conservation-of-momentum argument only works because the collision forces are internal to the two-block system and the net external force is zero. The 2022 Short FRQ Q1 (blocks connected over a pulley with a spring) tests the flip side. There, external forces like gravity and the spring do work on the system, so you track energy transfers instead of assuming conservation. In MCQs, expect stems like "momentum of the system is conserved because..." where the correct answer points to zero net external force, and trap answers blame internal forces. In FRQ justifications, explicitly naming your system and stating that the net external force on it is zero is exactly the kind of reasoning that earns points.

## External Forces vs Internal Forces

The difference is the system boundary, not the force itself. The normal force between two stacked blocks is internal if your system is both blocks, but external if your system is only the top block. Internal forces come in third-law pairs inside the system and cancel, so they can't change the system's total momentum or angular momentum. External forces have their reaction pair outside the system, so nothing cancels and the system's totals can change. Quick test: ask whether the object exerting the force is inside or outside your chosen system.

## Key Takeaways

- An external force is exerted on a system by something outside the system, and whether a force counts as external depends entirely on how you define the system.
- Only external forces can change a system's total momentum; internal forces cancel in Newton's third law pairs, which is why momentum is conserved in collisions when the net external force is zero.
- When you write Newton's second law for a whole system, only the net external force matters, because all internal forces have already canceled out.
- In simple harmonic motion, an external restoring force like a spring force pulls the object back toward the equilibrium position, driving the oscillation.
- The same logic applies to rotation in Units 5 and 6, where only external torques can change a rigid system's angular momentum or rotational kinetic energy.
- On FRQs, start by stating your system and identifying the external forces on it. That one sentence often anchors a conservation justification worth points.

## FAQs

### What is an external force in AP Physics 1?

An external force is a force exerted on your chosen system by an object outside that system, like friction from the floor acting on a block. External forces are the only forces that can change a system's total momentum, angular momentum, or mechanical energy.

### Is gravity an external force?

Usually yes, because the Earth is normally outside your system, so gravity is external and can change the system's momentum and energy. But if you include the Earth in your system (like in an Earth-ball system for energy problems), gravity becomes internal and gravitational potential energy belongs to the system.

### What's the difference between internal and external forces?

Internal forces act between objects inside the system and cancel as Newton's third law pairs, so they can't change the system's totals. External forces come from outside the system, so their reaction forces act on something outside and nothing cancels. The same force can be either one depending on your system boundary.

### Can internal forces ever change a system's total momentum?

No. Internal forces always come in equal-and-opposite third-law pairs within the system, so they sum to zero. That's exactly why momentum is conserved in collisions, like the 2024 FRQ where two blocks collide and the collision forces are internal to the two-block system.

### Why does momentum conservation require zero net external force?

Because the net external force equals the rate of change of the system's total momentum. If the net external force is zero, total momentum can't change, even though individual objects inside the system trade momentum through internal forces.

## Related Study Guides

- [5.2 Representations of Changes in Momentum](/ap-physics-1-revised/unit-5/representations-changes-momentum/study-guide/4Q0Vl38t8Nyyg3IrRXtf)
- [Unit 2 Overview: Dynamics](/ap-physics-1-revised/unit-2/review/study-guide/5wCs7oRuTfU4f61DUik4)
- [6.1 Period of Simple Harmonic Oscillators](/ap-physics-1-revised/unit-6/period-simple-harmonic-oscillators/study-guide/JdEVBczMfJ7h8rIl6TuZ)

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