Position, Velocity, and Acceleration

AP Physics 1· AP Physics 1 CED — Kinematics· 14 min read

1. Core Kinematic Quantities★☆☆☆☆⏱ 3 min

This topic forms the foundation of all kinematics, which makes up 10-16% of your total AP Physics 1 exam score. Concepts from this topic underlie every motion analysis question, from forces to circular motion, so accurate identification of quantities is critical.

📘 Definition

Core Kinematic Quantities

Key quantities are distinguished by whether they are scalar (only magnitude) or vector (magnitude + direction):

Example:

Position (vector): Location relative to a defined origin
Displacement (vector): Net change in position, $Δ x = x_{f} - x_{i}$
Distance (scalar): Total path length traveled
Velocity (vector): Rate of change of position
Speed (scalar): Magnitude of velocity
Acceleration (vector): Rate of change of velocity

📐 Worked Example

A hiker walks 3 km east along a straight trail, then turns around and walks 1 km west back toward the start. Find total distance traveled and total displacement, taking east as the positive direction.

1
Total distance is the scalar sum of all path segments:
2
$3\ \text{km} + 1\ \text{km} = 4\ \text{km}$
3
Displacement is the vector change in position, starting from $x_{i} = 0$ :
4
$x_f = 3 - 1 = 2\ \text{km}, \quad \Delta x = x_f - x_i = 2\ \text{km}$

2. Average vs Instantaneous Quantities★★☆☆☆⏱ 4 min

All kinematic quantities can be described as either average (measured over a finite time interval) or instantaneous (measured at a single moment in time). The definitions for average quantities are valid for all motion, whether acceleration is constant or changing.

\bar{v} = \frac{\Delta x}{\Delta t} = \frac{x_f - x_i}{t_f - t_i}, \quad \bar{a} = \frac{\Delta v}{\Delta t} = \frac{v_f - v_i}{t_f - t_i}

Instantaneous quantities are measured at a single moment in time. Graphically, instantaneous velocity is the slope of the tangent line to an $x$ - $t$ graph, and instantaneous acceleration is the slope of the tangent line to a $v$ - $t$ graph. On AP Physics 1 you read these slopes directly off a graph — no calculus required.

📐 Worked Example

A bicyclist moves along a straight path with position function $x (t) = 0.5 t^{2} + 2 t + 1$ , where $x$ is in meters and $t$ is in seconds. Find (a) the average velocity between $t = 1 s$ and $t = 5 s$ , and (b) the instantaneous velocity at $t = 3 s$ .

1
Calculate initial and final position for the interval:
2
$x(1) = 0.5(1)^2 + 2(1) + 1 = 3.5\ \text{m}, \quad x(5) = 0.5(25) + 2(5) + 1 = 23.5\ \text{m}$
3
Compute average velocity using the core definition:
4
$\Delta x = 23.5 - 3.5 = 20\ \text{m}, \quad \Delta t = 5 - 1 = 4\ \text{s}, \quad \bar{v} = \frac{20}{4} = 5\ \text{m/s}$
5
For the instantaneous velocity, recognize that this position function has the constant-acceleration form $x = x_{0} + v_{0} t + \frac{1}{2} a t^{2}$ . Reading off the coefficients gives $v_{0} = 2 m/s$ and $\frac{1}{2} a = 0.5$ , so $a = 1 m/s^{2}$ .
6
Apply the constant-acceleration equation $v = v_{0} + a t$ at $t = 3 s$ :
7
$v = v_0 + a t = 2 + (1)(3) = 5\ \text{m/s}$

3. Graphical Relationships Between Quantities★★☆☆☆⏱ 4 min

AP Physics 1 heavily tests graphical interpretation of kinematic quantities. The core relationships follow two simple rules:

Slope Rule: The slope of any kinematic graph equals the next quantity in the chain: $slope of x - t = v$ , $slope of v - t = a$
Area Rule: The net signed area under any kinematic graph equals the change in the previous quantity in the chain: $net area under v - t = Δ x$ , $net area under a - t = Δ v$

Area is signed: area above the time axis is positive, area below is negative, corresponding to positive or negative velocity/acceleration.

📐 Worked Example

A $v$ - $t$ graph for a toy car moving along a straight track has three segments: (1) $t = 0$ to $t = 2 s$ : horizontal line at $v = 4 m/s$ , (2) $t = 2 s$ to $t = 4 s$ : straight line from $v = 4 m/s$ to $v = 0$ , (3) $t = 4 s$ to $t = 6 s$ : straight line from $v = 0$ to $v = - 4 m/s$ . Find the total displacement of the car from $t = 0$ to $t = 6 s$ .

1
Calculate the signed area for each segment, since area under $v$ - $t$ equals displacement:
2
Segment 1 (0 to 2 s): Rectangle area:
3
$2\ \text{s} \times 4\ \text{m/s} = 8\ \text{m}$
4
Segment 2 (2 to 4 s): Triangle area:
5
$0.5 \times 2\ \text{s} \times 4\ \text{m/s} = 4\ \text{m}$
6
Segment 3 (4 to 6 s): Triangle below the axis, so area is negative:
7
$0.5 \times 2\ \text{s} \times (-4\ \text{m/s}) = -4\ \text{m}$
8
Sum the areas for total displacement:
9
$\Delta x_{\text{total}} = 8 + 4 - 4 = 8\ \text{m}$

4. Constant Acceleration Kinematic Equations★★★☆☆⏱ 3 min

For motion with constant acceleration (e.g., free fall near Earth's surface, constant braking), we can derive three simplified equations that make problem solving much faster. These equations only work when acceleration is constant — if acceleration changes, use graphical methods or the core definitions instead.

\begin{align} v &= v_0 + a t \tag{1} \\ \Delta x &= v_0 t + \frac{1}{2} a t^2 \tag{2} \\ v^2 &= v_0^2 + 2 a \Delta x \tag{3} \end{align}

Each equation omits one unknown quantity, so you can always select the equation that matches your known values to solve for the unknown in one step.

📐 Worked Example

A ball is thrown straight upward from ground level with an initial speed of $20 m/s$ . Take upward as positive, and acceleration due to gravity $a = - 10 m/s^{2}$ . Find the maximum height the ball reaches.

1
List all known and unknown quantities: initial velocity $v_{0} = 20 m/s$ , final velocity $v = 0 m/s$ (the ball stops momentarily at maximum height), acceleration $a = - 10 m/s^{2}$ , unknown is displacement $Δ x$ (maximum height).
2
Select the equation that omits time (our unknown): equation 3.
3
Substitute the known values into the equation:
4
$0^2 = (20)^2 + 2(-10)\Delta x$
5
Solve for $Δ x$ :
6
$0 = 400 - 20\Delta x \rightarrow 20\Delta x = 400 \rightarrow \Delta x = 20\ \text{m}$

5. AP Style Concept Check★★★☆☆⏱ 4 min

✓ Quick check

Test your understanding with these AP-style questions:

The velocity of an object moving in a straight line is shown on a $v$ - $t$ graph as a straight line rising from $2 m/s$ at $t = 0$ to $10 m/s$ at $t = 4 s$ . What is the instantaneous acceleration of the object at $t = 2 s$ ?
- $2 m/s^{2}$
- $4 m/s^{2}$
- $5 m/s^{2}$
- $8 m/s^{2}$

📐 Worked Example

A driver is traveling at $15 m/s$ on a suburban street when a cat runs into the road. The driver has a reaction time of $0.6 s$ (time between seeing the cat and pressing the brakes), after which the brakes provide a constant deceleration of $4.5 m/s^{2}$ . What is the total distance the car travels from the moment the driver sees the cat to when it stops completely?

1
Split the motion into two segments: reaction time (constant velocity, no acceleration) and braking (constant deceleration).
2
Reaction time segment distance:
3
$d_1 = v_0 t_{\text{react}} = 15 \times 0.6 = 9\ \text{m}$
4
For braking: $v_{0} = 15 m/s$ , $v = 0$ (stopped), $a = - 4.5 m/s^{2}$ :
5
Use the constant acceleration equation that omits time:
6
$0 = 15^2 + 2(-4.5)d_2 \rightarrow 0 = 225 - 9d_2 \rightarrow d_2 = 25\ \text{m}$
7
Total stopping distance:
8
$d_{\text{total}} = d_1 + d_2 = 9 + 25 = 34\ \text{m}$

6. Common Pitfalls

Wrong move:

Calculates average velocity as $(v_{i} + v_{f}) /2$ for motion with non-constant acceleration.

Why:

Students memorize this shortcut for constant acceleration and overgeneralize it to all motion.

Correct move:

Always calculate average velocity from the core definition $\overset{v}{ˉ} = Δ x /Δ t$ for any motion; only use the average-of-velocities shortcut when acceleration is explicitly constant.

Wrong move:

Interprets the y-value of a velocity-time graph as position.

Why:

Students mix up graph axes, confusing position and velocity quantities.

Correct move:

Before interpreting any kinematic graph, label each axis, then remind yourself: 'slope of current = next quantity, area of current = previous quantity'.

Wrong move:

Adds total path length to get displacement, giving a positive displacement when the object ends up left of its starting position.

Why:

Students confuse scalar distance with vector displacement.

Correct move:

Always calculate displacement as final position minus initial position, regardless of the path taken between them.

Wrong move:

Only takes the positive square root when solving $v^{2} = v_{0}^{2} + 2 a Δ x$ , even when the object is moving in the negative direction.

Why:

Students assume velocity is always positive, forgetting velocity is a signed vector.

Correct move:

After taking the square root, always check the direction of motion to select the correct sign for your final answer.

Wrong move:

Claims that zero acceleration means zero velocity.

Why:

Students confuse acceleration (rate of change of velocity) with velocity itself.

Correct move:

Always remember: zero acceleration means constant velocity (can be non-zero), zero velocity means instantaneous zero speed (can have non-zero acceleration, e.g., a ball at maximum height).

Wrong move:

Treats all area under a $v$ - $t$ graph as positive when calculating net displacement.

Why:

Students think area is always positive, ignoring that negative velocity produces negative displacement.

Correct move:

Assign a negative sign to all area that lies below the time axis before summing for net change.

7. Quick Reference Cheatsheet

Category	Formula	Notes
Displacement	$Δ x = x_{f} - x_{i}$	Vector net change; not equal to total distance
Average Velocity	$\overset{v}{ˉ} = Δ x /Δ t$	Valid for all motion, constant or not
Instantaneous Velocity	$v = slope of x - t$	Read off as the tangent slope on the graph; no calculus needed
Average Acceleration	$\overset{a}{ˉ} = Δ v /Δ t$	Valid for all motion
Instantaneous Acceleration	$a = slope of v - t$	Also equals the slope of the tangent to the $v$ - $t$ graph
Graphical Area Rule	$Δ x = net area under v - t; Δ v = net area under a - t$	Area below axis is negative, area above is positive
Constant Acceleration 1	$v = v_{0} + a t$	Valid only for constant $a$ ; omits $Δ x$
Constant Acceleration 2	$Δ x = v_{0} t + \frac{1}{2} a t^{2}$	Valid only for constant $a$ ; omits final velocity $v$
Constant Acceleration 3	$v^{2} = v_{0}^{2} + 2 a Δ x$	Valid only for constant $a$ ; omits time $t$

Going deeper

SyllabusOfficial AP Physics 1 CEDOfficial kinematics learning objectives

What's Next

Position, velocity, and acceleration are the foundational building blocks for all of kinematics, and for the entire AP Physics 1 course. Every topic that follows, from Newton’s laws of motion to energy, momentum, and circular motion, relies on your ability to correctly relate these three quantities to analyze motion. Without a solid understanding of how to interpret graphs of these quantities and apply the constant acceleration kinematic equations, you will struggle to set up and solve almost every free-response question on the exam, and many multiple-choice questions as well. Next, you will extend these 1-dimensional concepts to 2-dimensional motion, starting with projectile motion, where you separate horizontal and vertical motion into independent 1-dimensional kinematics problems that use all the rules you learned here.