Okay, smart folks. Time for the Teeming Millions to help me out here.
I remember my physics teacher way back in high school telling me that the term “accelerate” meant to change velocity.
She went on to say that “accelerate” could mean slowing down, because that was a change in velocity.
I asked my english teacher about this later and she promptly pulled out a dictionary and looked it up, pointing out to me that it meant to go faster, to speed up, change speed by going faster.
So, I thought, who is right and who is dumb?
Or are they both right, and the term means two different things in the two fields?
Is the term “decelerate” something used only in language, or is it used in science, too?
I never took any advanced sciences in college, so I don’t know!
I know someone knows (Alphagene, where are ya, when I need ya?) and I hope someone can tell me.
What’s the straight dope?
“Winners never quit and quitters never win, but those who never win and never quit are idiots.”
And
“This site is more addictive than caramel-covered crack!”
I think both your teachers were correct. The term just has different meanings in different fields.
:::suddenly remembers the word “theory” from all the evolution vs. creationism debate:::
As for “decelerate”, it’s just means negative acceleration in physics, for acceleration in the negative direction, whatever you define that to be. So yes, I suppose a scientist would use this term for reasons of convenience.
On a related note, here’s something you might find funny on this particular issue…
Maybe a little picky, but I wouldn’t put it quite that way. I would say deceleration would be whenever it opposes the direction of the velocity.
Say you have a particle traveling back and forth along a line that stretches left and right, left being the negative direction, right being positive. If the velocity is in the negative direction, and acceleration is positive, then you get “decelaration”.
I do agree that it’s correct to call it acceleration even if it slows the particle down.
Whew.
Thanks. I was considering questionin everything that I learned, on the basis that if two teachers could be wrong about one thing…
Thanks!
“Winners never quit and quitters never win, but those who never win and never quit are idiots.”
And
“This site is more addictive than caramel-covered crack!”
In physics, the choice of whether to call a change in speed “acceleration” or “deceleration” depends on an arbitrary choice of a reference frame. Physicists don’t like terms that change with regard to reference frames; it screws up their equations.
What is invariant in all reference frames is the energy value of the change in velocity, and the fact that the object changes its vector of motion. Therefore they formally define a term “acceleration” which can be applied in to any change of motion any reference frame. It simplifies the equations considerably!
Let’s start with velocity. In physics, velocity is a vector, which means that it has a direction attached. If you are driving a car and keeping the speedometer at 50, but make a turn, your velocity changes. Speed is the term used when direction is not a factor, which would be what your speedometer tells you.
In physics, acceleration is the change in speed divided by the elapsed time. This is calculated without regard for what the velocity is. A car that starts from rest and accelerates to 60 mph in 8.0 sec. has the same acceleration as a car heading in the opposite direction at 70 mph that slows down to 10 mph in 8.0 sec. Incidentally, according to my calculations, that is the same acceleration as a car driving at 60 mph around a circle of radius 448 ft. experiences.
In everyday usage, acceleration seems to be used to describe an increase in speed, and decceleration seems to be used for a decrease in speed. Although automobile enthusiasts do use the term “lateral acceleration” to describe the centripetal acceleration of a car as it travels around a circle. Generally these figures are given in terms of a factor of standard gravitational acceleration (~32.2 feet per second per second).
If you are in the car I mentioned above, your acceleration would be 60 mph / 8.0 sec., which works out to 11 ft. per sec. per sec. (60 mph ~= 88 fps). This would be .342 G’s, so the “centrifugal force” you would feel would be .342 times your weight.
One example I can think of to illustrate the concept of acceleration not being dependent on the direction of motion is if you throw a baseball into the air. Throughout its flight it is accelerating downward at 32.2 feet per second per second. If you’ve ever had to solve these sorts of problems, you would like this definition of acceleration, since it would be dumb to divide the problem into a decceleration half and an acceleration half.
So, how’s that for the long answer with much extraneous info. that doesn’t really explain the situation all that much better?
Almost, I think. Acceleration in Physics is the rate of change of velocity not the rate of change of speed.
This is the definition that you use in your examples.
The car going faster and the car slowing down have acceleration of the same magnitude but of opposite sense.
The other important factor about the baseball flight problem is that you can separate it into horizontal and vertical components so that, ignoring air friction, only the vertical component has any acceleration.
Russell
So is it generally agreed that “velocity” is a vector and “speed” is its magnitude? I must have missed that memo.
As to the OP, there are many words which have one meaning in common usage, but have taken on a somewhat different one in specialized fields. Your English and Physics teachers would also give you separate definitions of “energy” and “work”.
My sister, who works in the marketing group for a hospital company, once was writing something and asked me for another word for “tertiary”. I suggested “third”, not understanding why that would be so hard. Turns out that hospitals speak of “primary”, “secondary”, and “tertiary” care, and my sister of course knew the standard meanings of the first two, but was unaware that “tertiary” meant anything else outside of hospital lingo.
I don’t know if it’s generally agreed, but in physics velocity is a vector and speed is a scalar. In common usage, people don’t really care much about the difference. And in physics, acceleration is also a vector, and I don’t know of any word for rate of change in speed. In common usage, acceleration generally seems to be used for an increase in speed, except when talking about “lateral acceleration”, when the definition from physics applies.
On a related note, I seem to have miscalculated the radius of the circle in my above example. It should be 704 ft. Everything else is correct as far as I can tell.
The time derivative of acceleration is pretty widely known as “jerk”. The further derivatives have less commonly accepted names. I’m a snap, crackle, and pop advocate myself.
As far as velocity and speed go, the other posts have pretty much hit the nail on the head (velocity is a vector, speed is the scalar magnitude of velocity), but another difference between the two terms is that “velocity” sounds more technical, since in tecnical applications, it’s USUALLY the velocity you’re interested in. On the other hand, “escape velocity” (how fast you have to go to escape the gravity of a planet) should properly be called “escape SPEED”, as it’s independent of direction. Similarly, the velocity of light is not a constant, as it can go in any direction, but its speed is.
Moral of this story: Don’t try to sound smarter than you are.
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe”
–A. Einstein
So, to make a long, pedantic, drawn out story short, if I told my friend that:
“Acceleration doesn’t always mean to go faster, it simply means a change in velocity. So when you slow down, you are in fact accelerating.”
I wouldn’t be wrong would I? I don’t want to open my fat mouth on something that I think is right and look like a shmuck.
I do that enough as it is.
So, thanks for all the info, I shall put it to good use.
Heh heh heh.
“Winners never quit and quitters never win, but those who never win and never quit are idiots.”
And
“This site is more addictive than caramel-covered crack!”
