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How Fast Is Your Broomstick?

Everyday Einstein explains acceleration. Plus - how fast was Ron Weasley's Cleansweep flying?

By
Lee Falin, PhD,
February 21, 2014
Episode #089

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When I was younger, I had a couple of friends who were really into cars. You know the type, (you might even be the type), they carry around the latest issue of Car and Driver, they know the latest models of both the everyday sort of cars, as well as the fancy sports cars that most people have never heard of.

For some reason, none of the makes and models ever really stuck with me, but one of the things that I do remember is that each of the hot, new cars featured in these magazines had various statistics associated with them, designed to show off how much cooler they were than the other cars you might already know about. One of those statistics was the 0-60 stat, and it goes along with the topic I want to discuss today: acceleration.

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On Last Week’s Episode...

In last week's episode called, What's the Difference Between Speed and Velocity?, I mentioned that the words “speed” and “velocity” don’t always mean the same thing to scientists as they do to everyone else. To a scientist, speed is the measurement of how fast an object moves in a given length of time. So 60 miles per hour is a measurement of speed that tells us how many miles something travels each hour.

I also mentioned that velocity is slightly different than speed because it measures an object’s displacement over a certain amount of time. Displacement is how far something moved from where it started. I also said that velocity is a vector quantity. This is just a fancy way of saying that a velocity measurement has to include a direction as well as a number (or magnitude).

So if you hopped into your car and started driving, and an hour later you were 30 miles north of your house, we’d say that your average velocity was 30 mph North.

He’s Really Accelerating

With that brief review out of the way, we can move on to acceleration. When most people say that something is accelerating, they mean that it’s moving faster. However to a scientist, acceleration means only one thing: how much an object’s velocity changes in a given amount of time.

This may seem like nitpicking, but it’s a very important distinction for a couple of reasons. First, remember that velocity is a vector quantity, or a measurement with a direction. That means if you’re traveling with a velocity of 50mph North and suddenly turn around and go the opposite direction at the same speed, you’re now traveling with a velocity of 50 mph South. Your speed hasn’t changed, but your velocity has because you changed direction.

So to a scientist, if you changed your direction, even if you’re still going the same speed, you have still accelerated because acceleration is a measurement of your change in velocity.

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