Deceleration – I hate this word!

I have written before about how I teach physics, and how I love it. I have done this for 20 years (not counting what I did while still in college), and I love it.

But there are things I hate about physics and the world. I am talking about using words that add to the misconception people have about how physics works. One of these words is – DECELERATION.

I realize it is an official word. According to https://www.merriam-webster.com/dictionary/decelerate – deceleration is to reduce the speed of: slow down OR to decrease the rate of progress of OR to move at a decreasing speed

While I get this definition and I am fine with it in principle, my issue arises with the fact that people want to use it incorrectly at times as well. I find that students want to use deceleration (or decelerate) when they actually mean negative acceleration. Part of the issue comes from people not understanding what the word acceleration means. So, let’s start there.

Acceleration means a change in the velocity of an object. The key word there is change. Most people think it only means speeding up. But that is not what the definition of the word says. Acceleration is ANY change in the velocity of an object; this means: speeding up, changing direction, AND slowing down.

Now that we have that out of the way – when objects slow down they are decelerating, but really they are accelerating. Deceleration is really a VERY specific form of acceleration. Deceleration is not always negative acceleration either. This is the most common misunderstanding about the word, and the idea that gets confused when the word comes up. The negative sign comes from the direction of the acceleration, and therefore it means you could be speeding up in the negative direction and therefore you have negative acceleration, but you are not decelerating since you are not slowing down. Alternatively, when you are slowing down it doesn’t mean your acceleration must be negative, but you are decelerating. For example, you put your car in reverse and hit the accelerator in your car. The car speeds up in the negative direction (negative acceleration). Then you let off the accelerator and the car starts to slow down; since you are slowing down you are accelerating again, but in the opposite direction of your motion which is positive (therefore positive acceleration), but you are decelerating. Direction matters!

Or another example I like. You take a ball and roll it along a VERY smooth surface (assume little to no friction). Then that surface comes to an upward incline. The ball will roll up the incline, slowing down, until it comes to a stop, turns around and heads back down the incline speeding up as it descends. Why did the ball slow down on the way up – the force of gravity was acting on it. What caused it to turn around – the force of gravity acting on it. What caused it to speed up on the way down the incline – yes the force of gravity. The force acting on the ball never changed during the entire process of it going up, stopping, changing directions, and coming back down the incline. So, the rate of change of the velocity was always constant. Therefore, it was doing the same thing the entire time, and that thing was accelerating. It didn’t change from decelerating to accelerating. It didn’t change from a negative acceleration (slowing down) to a positive acceleration (speeding up). It had a constant acceleration the entire time that was directed “down” (and along) the incline.

I don’t allow this word in my classes because it just messes with student understanding. It is all just acceleration. You just have to pay attention to the direction.

One last thing, acceleration come from a net force acting on the object. Force equals mass times acceleration (not deceleration). That is Newton’s second (fixed after it was caught by my brother, thanks) law of motion. His law doesn’t change depending on what the object is doing. It is always defining forces with respect to the acceleration.

So, let’s just call it what it really is, acceleration, and stop using that other, stupid word.