Acceleration for Physics Dummies

Change in velocity

No matter the topic, to the uninitiated the specialized terminology of a discipline just looks like jargon. To scientists, for example, the word "theory" has a specific meaning; and it's not the meaning assumed by many non-scientists. In reality, these "jargon" meanings are needed to ensure accuracy when conveying an idea or concept. If a word has only one meaning, then the only way to misinterpret its use is to be unaware of that meaning. Unaware, for instance, like eHow.com contributor Athena Hessong, who made it abundantly clear that she was out of her scientific depth in "How to Calculate a Change in Velocity."¹

Let's open by defining "velocity." In general use it merely means speed, but to a physicist it is a vector quantity. "Vector" means that describing an object's velocity requires both a component of speed and a component of direction. That, unfortunately, is not what Hessong said in her introduction:

"Velocity measures how much an object's speed changes over a given time. "

Well, no: velocity measures how an object's position changes per unit time, not how much its "speed changes." That's sort of a description of acceleration, the rate of change of velocity. Given that harebrained daffynition of velocity, it'll be interesting to see how Athena said one might calculate change in velocity... so let's look. According to Hessong, you Subtract the beginning velocity from the ending velocity to find the change in velocity. Find the change in velocity using the acceleration by multiplying the acceleration by the time period to determine the change in velocity.

How to explain... how to explain... OK: you're calculating acceleration here (even if the word isn't in the question), and the classical physics solution is to divide the change in velocity by the change in time:

a = (Vf - Vi) / (Tf - Ti). 

Athena's step number 2 was a little closer (blind pigs, acorns...). Let's look at the units: acceleration is in m/s², so multiplying that by time in seconds would give you m/s, the speed component of velocity (a scalar rather than a vector).

Hessong's inability to address the question made it clear that she was saddled with a raging case of scientific illiteracy (does one take introductory physics to get a BA in History? probably not...). Due to this educational deficiency, she thought that a "change in velocity" is merely the difference between two speedometer readings. It's not (damned scientific "jargon") – and that's why she's picking up another Dumbass of the Day award that, to be honest, we had already awarded to her on the basis of the first sentence in her post. She should get another one for the last sentence in her introduction...

"Use the change in velocity to find the object's acceleration which divides the time by the change in velocity..."

Yes: she really did say that.

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. The URL was   ehow.com/how_6207363_calculate-constant-speed.html

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