What’s the Difference Between Speed, Velocity, and Acceleration?
These three terms get mixed up because they all describe motion. Here’s the cleanest version: speed is how fast, velocity is how fast PLUS direction, acceleration is how velocity changes.
Quick answer
Speed = magnitude only (e.g., 30 mph). Velocity = magnitude + direction (e.g., 30 mph east). Acceleration = how velocity changes over time. All three have specific units.
Speed
Speed is a scalar — a number with units like m/s, mph, km/h. Examples: a car going 60 mph, a runner moving at 5 m/s.
Formula: speed = distance / time.
Velocity
Velocity is a vector — it has magnitude AND direction. “30 mph north” is a velocity. “30 mph” alone is just speed.
If you walk 30 mph for 2 hours then turn around and walk back, your AVERAGE speed is 30 mph (you covered distance). Your AVERAGE velocity over the full trip is 0 (you ended where you started).
Acceleration
Acceleration is how velocity changes per second. If your velocity changes from 0 to 60 mph in 6 seconds, your acceleration is 10 mph/s.
Units: m/s² or mph/s.
Worked examples
Example 1 — speed
A runner covers 400 m in 50 seconds. Speed = 400/50 = 8 m/s.
Example 2 — velocity
A plane flies 500 km east in 1 hour. Velocity = 500 km/h east.
Example 3 — acceleration
A car goes from 0 to 30 m/s in 6 seconds. Acceleration = (30 – 0)/6 = 5 m/s².
Why this matters for physics
Newton’s second law is F = ma, where a is ACCELERATION (not speed). The forces on an object change its acceleration, which changes its velocity, which changes its position. Speed alone can’t tell you what forces are involved.
Common mistakes
- Using “velocity” when you mean “speed.”
- Forgetting that direction is part of velocity.
- Thinking constant speed means no acceleration (circular motion has constant speed but changing velocity).
- Mixing units (m/s with km/h).