# This Is the Definition of Velocity That You Should Use In Physics

If you are teaching or using physics, please DO NOT use this definition for the velocity in one dimension (it’s a bad idea):

“Velocity = distance divided by time.”

OK, technically there are some cases where this would be fine — but it leads to the really terrible version of velocity that looks like this:

This is really wrong. Oh sure, it can sometimes give you the correct answer — but even a broken clock is right twice a day. I mean I see why people use this equation. If they use x for distance and t for time then it’s just the same thing as distance divided by time. It also has the correct units for velocity. But it’s still wrong.

The Best Definition

Let’s get to it. Here is the best definition of the velocity in one dimension (for an object moving along the x-axis).

First, I will point out the “avg” part of this. This equation is for the average velocity in the x-direction. If you have an object with a changing velocity (like a car with a rocket pushing it forward) then the velocity is not constant — but this equation still works as long as you realize it’s the average velocity.

Second, the important part of this is the Greek letter, Δ (Delta). We use this to mean “change in”…

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Physics faculty, science blogger of all things geek. Technical Consultant for CBS MacGyver and MythBusters. WIRED blogger.