SAT / PSAT
SAT / PSAT Prep
History & Social Science
AP World History AP US History AP European History AP Human Geography AP US Government & Politics AP Psychology AP Macroeconomics AP Microeconomics
English
AP English Language & Composition AP English Literature & Composition
Math & Computer Science
AP Calculus AB/BC AP Precalculus AP Statistics AP Computer Science A AP Computer Science Principles
Sciences
AP Biology AP Chemistry AP Environmental Science AP Physics 1 AP Physics 2
World Languages & Arts
AP Spanish Language AP Art History AP Music Theory Start studying →
🏃 Unit 1 · Kinematics 🗂 Flashcards 🗺 Cheat Sheet ⭐ Essentials 🎙 Podcast 🎨 Visual Review 📝 MC Practice ✎ FRQ Practice

AP Physics 1 Unit 1 Essentials

The must-know terms and big ideas for Unit 1: Kinematics. Every vocabulary word and concept you need to master, from scalars and vectors to projectile motion.

← Back to Unit 1 hub
Big Idea 1
Motion is described by position, velocity, and acceleration — and they're linked through time
Velocity is how fast position changes; acceleration is how fast velocity changes. These three quantities are the entire vocabulary of motion. The kinematic equations connect them mathematically, and motion graphs show their relationships visually — slope of position-time gives velocity, slope of velocity-time gives acceleration, and the area under a velocity-time graph gives displacement.
Position Velocity Acceleration Graphs
Big Idea 2
Direction matters — vectors are different from scalars
Distance and speed only have size — they're scalars. Displacement, velocity, and acceleration have BOTH size and direction — they're vectors. Mixing these up is the most common Unit 1 mistake. A car driving in a circle at constant speed is STILL accelerating because its direction is changing. In one dimension, you can use positive and negative signs to show direction.
Scalars vs. Vectors Direction Components
Big Idea 3
Two-dimensional motion is really just two one-dimensional problems
In 2D, the horizontal and vertical motions are completely independent. Projectile motion has zero acceleration horizontally (constant velocity) but constant downward acceleration (g) vertically (free fall). The trick is: solve each direction separately using the kinematic equations, then combine. Time is the only thing the two dimensions share.
Projectile Motion 2D Motion Components
🔍
Kinematics
The study of motion without considering what causes it. You describe HOW things move (position, velocity, acceleration) without worrying about forces — that comes in Unit 2.
Foundation
Scalar
A quantity that has only magnitude (size), with no direction. Examples: distance, speed, time, mass, temperature. Always positive.
1.1 Scalars & Vectors
Vector
A quantity that has both magnitude AND direction. Examples: position, displacement, velocity, acceleration. Drawn as an arrow whose length shows magnitude and whose arrowhead shows direction.
1.1 Scalars & Vectors
Magnitude
The size or amount of a quantity, ignoring direction. Always positive. For a vector, the magnitude is the length of the arrow.
1.1 Scalars & Vectors
Vector Sum (1D)
Adding vectors that point along the same line. Opposite directions get opposite signs, so "5 m east + 3 m west" becomes +5 + (−3) = +2 m east.
1.1 Scalars & Vectors
Position
Where an object is located, measured from a chosen reference point (origin). A vector quantity. Symbol: x (in one dimension).
1.2 Displacement, Velocity, Acceleration
Distance
The total path length traveled by an object. A scalar. Walking 3 m east then 3 m west = 6 m distance (even though you end up where you started).
1.2 Displacement, Velocity, Acceleration
Displacement
The straight-line change in position from start to finish, with direction. A vector. Walking 3 m east then 3 m west = 0 m displacement. This is the most common scalar-vs-vector trap on the exam.
1.2 Displacement, Velocity, Acceleration
Speed
How fast something moves (distance divided by time). A scalar. Always positive. Units: m/s.
1.2 Displacement, Velocity, Acceleration
Velocity
How fast AND in what direction something moves (displacement divided by time). A vector. Can be positive or negative. A car going 60 mph east and one going 60 mph west have the same speed but different velocities.
1.2 Displacement, Velocity, Acceleration
Average Velocity
Total displacement divided by total time interval. Tells you the overall motion, not what happened at any single moment.
1.2 Displacement, Velocity, Acceleration
Instantaneous Velocity
Velocity at a single instant in time. On a position-time graph, it's the slope of the line tangent to the curve at that point.
1.2 Displacement, Velocity, Acceleration
Acceleration
The rate at which velocity changes (change in velocity divided by time). A vector. An object accelerates if its speed OR its direction is changing. Units: m/s².
1.2 Displacement, Velocity, Acceleration
Object Model
A simplification where you treat an entire object as a single point with mass, ignoring its size and shape. Works great for a baseball flying through the air.
1.2 Displacement, Velocity, Acceleration
Motion Diagram
A visual showing an object's position at evenly spaced time intervals — like a strobe photo. Useful for picturing how speed and direction change over time.
1.3 Representing Motion
Kinematic Equations
The "Big Three" equations for motion with constant acceleration: v = v₀ + at; x = x₀ + v₀t + ½at²; v² = v₀² + 2a(x − x₀). They let you solve for unknowns when acceleration is constant.
1.3 Representing Motion
Acceleration Due to Gravity (g)
The downward acceleration of any object in free fall near Earth's surface. AP Physics 1 uses g ≈ 10 m/s² (9.8 or 9.81 m/s² are also accepted).
1.3 Representing Motion
Free Fall
Motion where gravity is the only force acting on an object. The object accelerates downward at g, regardless of its mass. Air resistance is ignored.
1.3 Representing Motion
Position-Time Graph
A graph of position vs. time. The SLOPE at any point equals the velocity at that moment. A flat line means at rest; a steep line means fast motion.
1.3 Representing Motion
Velocity-Time Graph
A graph of velocity vs. time. The SLOPE = acceleration. The AREA UNDER THE CURVE = displacement. Crossing zero means changing direction.
1.3 Representing Motion
Acceleration-Time Graph
A graph of acceleration vs. time. The AREA UNDER THE CURVE = change in velocity over that time interval.
1.3 Representing Motion
Reference Frame
The viewpoint from which an observer measures positions and velocities. The same motion can look different from different reference frames — but they all agree on the acceleration.
1.4 Reference Frames
Inertial Reference Frame
A reference frame that isn't accelerating — one where Newton's first law works. For AP Physics 1, assume frames are inertial unless told otherwise.
1.4 Reference Frames
Relative Velocity
The velocity of an object measured from a particular reference frame. If a person walks 2 m/s on a train moving 30 m/s, their velocity relative to the ground is 32 m/s. AP Physics 1 tests relative velocity in one dimension only.
1.4 Reference Frames
Vector Components
The pieces of a vector along the x and y axes. Any vector can be broken into perpendicular x- and y-components: x-component = magnitude × cos(θ); y-component = magnitude × sin(θ).
1.5 Vectors & 2D Motion
Resultant
The single vector that represents the sum of two or more vectors. Found by adding components separately, then using the Pythagorean theorem for magnitude.
1.5 Vectors & 2D Motion
Projectile Motion
Motion of an object launched into the air, moving only under gravity. Horizontal motion is constant velocity (zero acceleration); vertical motion is free fall (constant downward acceleration g). The two motions are independent but share the same time.
1.5 Vectors & 2D Motion
Trajectory
The curved path a projectile follows — a parabola, when air resistance is ignored.
1.5 Vectors & 2D Motion