Describing Motion: Position, Speed, and Velocity

Overview

Have you ever watched a car drive down the street or a friend run across a field? To describe how things move, scientists use special words: position, speed, and velocity. Understanding these terms helps us talk about motion clearly and prepares us for learning about forces and acceleration later.

1. Position: Where Are You?

Position tells us where something is. Imagine a number line:

|---|---|---|---|---|---|---|
0   1   2   3   4   5   6

If you stand at 3, your position is 3 meters from the start (0).

Position is usually measured in meters (m).
It needs a reference point (where you start counting from).

Example:

"The ball is at position 5 meters."

2. Speed: How Fast?

Speed tells us how fast something is moving, but not the direction.

Speed = Distance traveled / Time taken
The unit is meters per second ($m/s$).

Formula:

$$ \text{Speed} = \frac{\text{Distance}}{\text{Time}} $$

Example:

If you run 10 meters in 2 seconds:

$$ \text{Speed} = \frac{10\ m}{2\ s} = 5\ m/s $$

3. Velocity: Speed with Direction

Velocity is like speed, but it also tells us the direction.

Velocity = Displacement / Time
Displacement is the straight-line distance from start to finish, including direction.
The unit is also meters per second ($m/s$), but we add the direction (e.g., 5 m/s east).

Formula:

$$ \text{Velocity} = \frac{\text{Displacement}}{\text{Time}} $$

Example:

If you walk 10 meters east in 2 seconds:

$$ \text{Velocity} = \frac{10\ m\ \text{east}}{2\ s} = 5\ m/s\ \text{east} $$

4. Speed vs. Velocity: What's the Difference?

Speed: Only tells how fast (no direction).
Velocity: Tells how fast and which way.

Common Confusion:

If you run in a circle and end up where you started, your speed was not zero, but your velocity is zero (because your displacement is zero).

5. Practice: Try It Yourself!

If a cyclist travels 20 meters north in 4 seconds, what is their speed and velocity?
- Speed: $\frac{20\ m}{4\ s} = 5\ m/s$
- Velocity: $5\ m/s$ north
If you walk 6 meters east, then 6 meters west in 6 seconds, what is your speed and velocity?
- Speed: $\frac{12\ m}{6\ s} = 2\ m/s$
- Velocity: $\frac{0\ m}{6\ s} = 0\ m/s$ (because you end up where you started)

Summary Table

Concept	What it tells us	Formula	Unit
Position	Where something is	---	meters (m)
Speed	How fast (no direction)	$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$	m/s
Velocity	How fast & which way	$\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}$	m/s + direction

Key Takeaways

Position is where you are.
Speed is how fast you move.
Velocity is how fast and in which direction you move.
Speed and velocity are not always the same!

Ready to learn about what causes motion? Next up: Forces and Acceleration!

Understanding Speed and Velocity