---
title: "Constant Acceleration — AP Physics 1 Definition & Guide"
description: "Constant acceleration means velocity changes by the same amount every second. It unlocks the kinematics equations like v = v₀ + at and powers free-fall problems."
canonical: "https://fiveable.me/ap-physics-1-revised/key-terms/constant-acceleration"
type: "key-term"
subject: "AP Physics 1"
---

# Constant Acceleration — AP Physics 1 Definition & Guide

## Definition

Constant acceleration is motion where an object's velocity changes by the same amount in each equal time interval, so acceleration has a fixed magnitude and direction. In AP Physics 1, it's the condition that lets you use the kinematics equations, like v⃗ = v⃗₀ + a⃗t (Topic 1.1, Unit 1).

## What It Is

Constant acceleration means the [velocity](/ap-physics-1-revised/unit-1/2-displacement-velocity-and-acceleration/study-guide/HyscWF2F28uakfpc "fv-autolink") changes at a steady rate. If a car gains 3 m/s of [speed](/ap-physics-1-revised/key-terms/speed "fv-autolink") every second, its acceleration is a constant 3 m/s². The size of the velocity change is the same for every one-second slice of the motion.

Because acceleration is a vector, "constant" means both the magnitude AND the [direction](/ap-physics-1-revised/unit-1/4-reference-frames-and-relative-motion/study-guide/iTcYEEULwbQlf2nW "fv-autolink") stay fixed. That's exactly the condition baked into the equation from Topic 1.1, v⃗ = v⃗₀ + a⃗t. This formula only works when a⃗ doesn't change. On a velocity-time graph, constant acceleration shows up as a straight line, and the slope of that line is the acceleration. Free fall is the classic example, where every object near Earth's surface accelerates downward at about 9.8 m/s² regardless of its mass.

## Why It Matters

Constant acceleration lives in **Topic 1.1 ([Position](/ap-physics-1-revised/key-terms/position "fv-autolink"), Velocity, and Acceleration)** in **[Unit 1](/ap-physics-1-revised/unit-1 "fv-autolink"): Kinematics**, supporting learning objective **1.1.A**, which asks you to describe vector quantities like acceleration with both magnitude and direction. The relevant equation v⃗ = v⃗₀ + a⃗t comes straight from that learning objective's essential knowledge. It also leans on **1.1.B**, since in one dimension you handle direction with plus and minus signs (a ball thrown up with positive velocity has negative acceleration the whole flight). Almost every kinematics calculation you do in Unit 1, and later in projectile and dynamics problems, assumes constant acceleration. Recognizing when that assumption holds (and when it breaks) is half the battle on the exam.

## Connections

### Acceleration (Unit 1)

Acceleration is the rate of change of velocity. Constant acceleration is just the special case where that rate never changes, and it's the only case where the standard [kinematics equations](/ap-physics-1-revised/key-terms/kinematics-equations "fv-autolink") are valid.

### [Final Velocity (Unit 1)](/ap-physics-1-revised/key-terms/final-velocity)

Under constant acceleration, [final velocity](/ap-physics-1-revised/key-terms/final-velocity "fv-autolink") is completely predictable. Start with v₀, add a·t, done. That's the whole logic of v⃗ = v⃗₀ + a⃗t.

### [Vector Quantity (Unit 1)](/ap-physics-1-revised/key-terms/vector-quantity)

Acceleration is a [vector](/ap-physics-1-revised/key-terms/vector "fv-autolink"), so "constant" means the direction is locked too. An object moving in a circle at steady speed does NOT have constant acceleration because the direction of a⃗ keeps changing.

### Displacement (Unit 1)

With constant acceleration, displacement is the area under a straight-line velocity-time graph, which is why position-time graphs for this motion are parabolas instead of straight lines.

## On the AP Exam

No released FRQ uses the phrase "constant acceleration" as a term to define, but the concept is everywhere in Unit 1 questions. Multiple-choice stems test whether you can spot constant acceleration from a graph (straight line on velocity-time, curved parabola on position-time) or from data showing equal velocity changes over equal time intervals. You'll also be asked to apply v⃗ = v⃗₀ + a⃗t with correct signs, like recognizing that a ball at the top of its arc has zero velocity but still has acceleration of -9.8 m/s². FRQs reward you for stating the assumption explicitly. Writing "since acceleration is constant, I can use the kinematic equations" is exactly the kind of justification graders look for in experimental design and paragraph-response questions.

## Constant Acceleration vs Constant Velocity

Constant velocity means velocity isn't changing at all, so acceleration is zero. Constant acceleration means velocity IS changing, just at a steady rate. A car cruising at 60 mph has constant velocity. A car speeding up by 5 mph every second has constant acceleration. On graphs, constant velocity is a flat line on a v-t graph, while constant acceleration is a sloped straight line. Mixing these up is one of the most common point-losers in Unit 1.

## Key Takeaways

- Constant acceleration means velocity changes by the same amount in every equal time interval, in both magnitude and direction.
- The equation v⃗ = v⃗₀ + a⃗t from Topic 1.1 only works when acceleration is constant.
- On a velocity-time graph, constant acceleration looks like a straight line, and the slope of that line equals the acceleration.
- In one dimension, direction is handled with signs, so an object can have positive velocity and negative acceleration at the same time (that's just slowing down).
- Free fall is constant acceleration of about 9.8 m/s² downward, and it stays 9.8 m/s² even at the top of the path where velocity is zero.
- Constant acceleration does not mean constant velocity; if acceleration is constant and nonzero, the velocity is always changing.

## FAQs

### What is constant acceleration in AP Physics 1?

It's motion where an object's velocity changes by the same amount during every equal time interval, meaning acceleration keeps a fixed magnitude and direction. It's the condition required to use kinematics equations like v⃗ = v⃗₀ + a⃗t in Unit 1.

### Does constant acceleration mean constant speed?

No, it's basically the opposite. Constant nonzero acceleration means the velocity is changing at a steady rate, so the speed is constantly increasing or decreasing. Constant speed in a straight line means acceleration is zero.

### How is constant acceleration different from constant velocity?

Constant velocity means zero acceleration and a flat line on a velocity-time graph. Constant acceleration means velocity changes at a steady rate, giving a sloped straight line on a velocity-time graph. Both produce straight v-t lines, but only constant velocity gives a horizontal one.

### Is acceleration constant at the top of a projectile's path?

Yes. The velocity hits zero at the peak, but the acceleration stays a constant 9.8 m/s² downward the entire flight. Saying acceleration is zero at the top is one of the most common wrong answers on AP multiple choice.

### How do you tell if acceleration is constant from a graph?

Check the velocity-time graph. If it's a straight line (any slope), acceleration is constant and equals the slope. On a position-time graph, constant acceleration shows up as a parabola, not a straight line.

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