AP Physics 1 Unit 1: Kinematics

TLDR

• Kinematics describes motion without explaining the cause.

• You must interpret motion using equations, graphs, and words.

• Constant acceleration is assumed in most AP problems.

• Graph analysis is tested as heavily as calculations.

• Horizontal and vertical motion are analyzed separately.

Why This Unit Matters

Unit 1 is the foundation of all AP Physics 1. Every later unit, including forces, energy, momentum, and rotation, assumes you understand how motion works.

On the AP exam, kinematics is not just about equations. It tests whether you can reason about motion, interpret graphs, and explain physical behavior clearly. Many students lose points not from math errors, but from weak explanations and misread graphs.

Mastering this unit early makes the rest of the course significantly easier.

1. Describing Motion

Kinematics answers one question: How does an object move?

To describe motion, you need four core quantities:

• Position (x): where the object is relative to an origin

• Displacement (Δx): change in position, includes direction

• Velocity (v): rate of change of position

• Acceleration (a): rate of change of velocity

Velocity and acceleration are vectors. Direction matters on the AP exam.

2. Average vs Instantaneous Quantities

The AP exam frequently tests the difference between averages and instantaneous values.

• Average velocity: Δx ÷ Δt

• Instantaneous velocity: velocity at a specific moment

Instantaneous velocity is not calculated using averages. It is found by:

• The slope of a position vs time graph

• The velocity value at a specific time on a velocity vs time graph

This distinction is commonly tested conceptually.

3. Motion Graphs

Graphs are one of the most important skills in Unit 1.

Position vs Time (x–t)

• Slope represents velocity

• Curved line means acceleration

• Steeper slope means greater speed

Velocity vs Time (v–t)

• Slope represents acceleration

• Area under the curve represents displacement

• Horizontal line means constant velocity

Acceleration vs Time (a–t)

• Area represents change in velocity

• Constant acceleration appears as a flat line

Misreading slopes and areas is one of the biggest sources of lost points.

4. Constant Acceleration Equations

When acceleration is constant, motion can be described using kinematic equations.

Common equations used on the AP exam:

• v = v₀ + at

• Δx = v₀t + ½at²

• v² = v₀² + 2aΔx

• Δx = ½(v + v₀)t

Each equation applies only when acceleration is constant.

You must choose equations based on the known variables, not memorization.

5. Free Fall Motion

Free fall is vertical motion under gravity only.

Key ideas:

• Acceleration is constant and equal to g

• g = 9.8 m/s² downward

• Objects moving up and down experience the same acceleration

At the top of a trajectory:

• Velocity is zero

• Acceleration is still −g

Many students incorrectly assume acceleration becomes zero.

6. Two Dimensional Motion and Projectiles

Projectile motion combines horizontal and vertical motion.

Rules to remember:

• Horizontal and vertical motions are independent

• Horizontal acceleration is zero

• Vertical acceleration is −g

Horizontal motion uses constant velocity equations.
Vertical motion uses free fall equations.

Time connects the two directions.

Tutor Tip

When explaining motion on free response questions, always separate horizontal and vertical reasoning. This shows clear physics thinking and earns explanation points even if calculations are incomplete.

Common Pitfalls

• Confusing speed with velocity

• Ignoring direction signs

• Forgetting that acceleration exists even when velocity is zero

• Mixing up graph slopes and areas

• Using kinematic equations when acceleration is not constant

• Assuming horizontal velocity changes in projectile motion

• Forgetting units

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