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# Can an object be accelerating and yet -not- moving?

maxdancona

1
Fri 22 Feb, 2013 04:09 pm
@dalehileman,
Quote:
I suppose I might carry an instrument which when it reads zero tells me I'm stopped

There is no instrument that will tell you if you are stopped. This is scientifically impossible unless there is something to compare with... and then whatever you are comparing with may not be stopped.

But I am probably getting myself in trouble by adding something that is counter-intuitive even though it is scientifically correct and logically based.

The problem with intuition is that it always gets in the way of understanding.
maxdancona

1
Fri 22 Feb, 2013 04:11 pm
There is an interesting amusement park ride that let's you feel acceleration when you are stopped.

They strap you into a chair that shoots straight upward. Then at the top it pulls you back downward. At the very top there is an instant where you aren't moving, but boy do you feel the acceleration in the pit of your stomach.
0 Replies

dalehileman

1
Fri 22 Feb, 2013 04:32 pm
@maxdancona,
I suppose I might carry an instrument which when it reads zero tells me I'm stopped

Quote:
There is no instrument that will tell you if you are stopped.
I suppose I should have said, "when I'm not accelerating"

Quote:
But I am probably getting myself in trouble by adding something that is counter-intuitive even though it is scientifically correct and logically based.
I don't see how you could get in trouble as our difference seems largely semantic. I look at the word "accelerate" in a more commonplace way where you're referring to a mathematical process

Quote:
At the very top there is an instant where you aren't moving, but boy do you feel the acceleration in the pit of your stomach.
Well I probably shouldn't have used the term "feeling," since reaction obviously isn't instantaneous. I'd assume even though you're not stopped by gravity that there's still an instant of zero acceleration

Mind you I can't substantiate this assertion , it's just my old, unreliable Intuition at work. In the case of something tossed up in the air then coming back down I see the graph of acceleration as two hills of about the same size and shape pushed together so their bottoms meet, the point of no acceleration

..where in the case of the rollercoaster the hill to the right is much higher and its leading edge much steeper

Some day I shall have to learn how to draw digitally

Maybe we're somehow both wrong, as it seems like we have lost our other participants who've evidently decided we're both cuckoos
maxdancona

1
Fri 22 Feb, 2013 04:57 pm
@dalehileman,
First of all, we are not both wrong-- at least not mathematically or scientifically.

There is a mathematical definition of acceleration and a mathematical definition of velocity. Oddly enough, the human body can sense an acceleration (i.e. a condition that meets the mathematical definition). The human body can not sense velocity.

I am curious, how do you define acceleration if you reject the mathematical definition? If you are basing it on the feeling then you will largely correspond to mathematical acceleration.

However any definition of velocity or speed based on feeling doesn't correspond to the mathematical definition.

At any rate, what I am saying is absolutely correct in any science classroom or laboratory.

If you reject the scientific or mathematical understanding of acceleration and velocity then anything can be "true".
dalehileman

1
Fri 22 Feb, 2013 05:01 pm
@dalehileman,
Oops a2k cut me off again in the middle of a thought

"in the case of the rollercoaster" I should have said "Superman Ride " or "Tower of Power"

Exact terminology of supreme importance with such an impasse in which Max and I are engaged

Help, somebody who understands this stuff, at least better than I
0 Replies

dalehileman

1
Fri 22 Feb, 2013 05:11 pm
@maxdancona,
Quote:
There is a mathematical definition of acceleration and a mathematical definition of velocity.
Yes and I can understand how math can describe the moment of zero acceleration as a moment of acceleration

Quote:
Oddly enough, the human body can sense an acceleration
Yes as you have so correctly pointed out, except in the case of quick reversal

Quote:
The human body can not sense velocity.
True, as confirmed by our old friend Einstein

Quote:
I am curious, how do you define acceleration if you reject the mathematical definition?
Apparently I can't. However, I've emailed a few buddies who've studied that sort of stuff and maybe they'll come up with something

…that will doubtless prove me dead wrong

Quote:
If you are basing it on the feeling then you will largely correspond to mathematical acceleration.
This seems to contradict some of your earlier assertions

..at least according to my own Intuition

Quote:
However any definition of velocity or speed based on feeling doesn't correspond to the mathematical definition.
Of course not but how does this bear on our issue

Quote:
At any rate, what I am saying is absolutely correct in any science classroom or laboratory.
That there's acceleration at zero acceleration

Quote:
If you reject the scientific or mathematical understanding of acceleration and velocity then anything can be "true".
I haven't rejected it
0 Replies

engineer

3
Fri 22 Feb, 2013 09:30 pm
@dalehileman,
Quote:
Mind you I can't substantiate this assertion , it's just my old, unreliable Intuition at work. In the case of something tossed up in the air then coming back down I see the graph of acceleration as two hills of about the same size and shape pushed together so their bottoms meet, the point of no acceleration

Yet the answer is that acceleration is constant the entire time with no change from the time it leaves your hand until the time it hits the ground.
rosborne979

1
Fri 22 Feb, 2013 10:19 pm
@engineer,
That was almost poetic. Nicely done.
0 Replies

Thomas

1
Sat 23 Feb, 2013 12:11 am
@becquerel,
becquerel wrote:
But if "in that moment" the object is accelerating, how is there a chance it isn't moving?

Acceleration is a change in speed. When the speed of an object changes from a negative amount to a positive amount, there has to be a moment in between where the object is accelerating but not moving. For example, a pendulum at the outermost point of its swing is not moving, but it is accelerating.

Does that help?
dalehileman

1
Sat 23 Feb, 2013 05:29 am
@engineer,
Quote:
Yet the answer is that acceleration is constant the entire time with no change from the time it leaves your hand until the time it hits the ground.
Eng, Max, you fellas could be absolutely right, at least from a purely mathematical perspective. It's 3 a.m.; woke up an hour ago thinking about it but now totally confused, beginning to doubt my Intuition

But here's what it left me with, for what it's worth, which might not be very much: Looking at as a feeling rather than as a math function I see it not as two hills--I had forgotten it has a polarity--but approximately as one cycle of a sine wave

Of course the peak of the positive half-cycle is the moment of greatest force in your heave

Then I envision a moment of weightlessness where the positive half-cycle has returned to zero; then assuming you catch me on the way down, the negative half-cycle where again its peak is where I feel heaviest to you

Maybe it's merely a semantic issue where I graph the entire trip whereas you're graphing just the time I'm free of your hands. Still Intuition insists that where the graph passed through zero I felt zero acceleration

But like I said, guys, yes I could be totally offbase, but it's been very interesting chat
0 Replies

dalehileman

1
Sat 23 Feb, 2013 05:44 am
@Thomas,
Quote:
Acceleration is a change in speed. ……. not moving, but it is accelerating.
Does that help?
Not really Tom but thanks. See my posting above

Incidentally if it's no imposition who is becquerel and what did he have to say--

dalehileman

1
Sat 23 Feb, 2013 12:57 pm
@dalehileman,
10:45 a.m. Not arguing guys, just speculation:

Let's ignore the positive half-cycle and assume instead I don't need your help when I jump off a tall building. I am holding a ball bearing to judge whether I'm accelerating because I don't trust my other bodily feelings. At the moment I jump doesn't it become lighter, weighing virtually nothing on the way down (that's with the "negative half cycle" starting down too). So I call that negative acceleration

Let's not even consider your catching me or me hitting the ground

So at that moment of zero acceleration can't I say the acceleration was zero, no acceleration

Thus isn't our impasse merely semantical: Because the duration of my moment of no acceleration is zero, from your theoretical abstract transcendental mathematical point of view you're right, it's still accelerating but from my Intuitional point of view it wasn't
maxdancona

1
Sat 23 Feb, 2013 01:48 pm
@dalehileman,
First of all, you are wrong.

Engineer and I both have knowledge of physics. There isn't any sense in calling this an impasse when there is a fairly simple mathematically correct answer and several people with math-based degrees are telling you that you are wrong.

Now let's answer the question...

Quote:
So at that moment of zero acceleration can't I say the acceleration was zero, no acceleration

Zero acceleration means the velocity doesn't change. If the velocity is zero, and the velocity doesn't change then the velocity stays zero forever and you will hang in the air like a cartoon character.

There is no moment of zero acceleration. There is an "instant" (a better term than momen) of zero velocity, but not of acceleration.

Anyone with a basic high school understanding of physics will tell you the same thing (and several of us here have quite a bit deeper understanding than that).
dalehileman

1
Sat 23 Feb, 2013 02:31 pm
@maxdancona,
Quote:
Anyone with a basic high school understanding of physics will tell you the same thing
Very likely Max you're right but we don't seem to attract much comment one way or t'other
Thomas

1
Sat 23 Feb, 2013 03:42 pm
@dalehileman,
dalehileman wrote:
Very likely Max you're right but we don't seem to attract much comment one way or t'other

So?
0 Replies

engineer

1
Sat 23 Feb, 2013 04:47 pm
@dalehileman,
Consider this: while the ball is in flight, the only force acting on it is gravity. If acceleration is changing then gravity must be changing. Do you think gravity is changing during the ball's flight?
dalehileman

1
Sat 23 Feb, 2013 05:16 pm
@engineer,
Sorry Eng, I'm in a state of virtual rigour
contrex

1
Sat 23 Feb, 2013 06:10 pm
That remark about "any high school student" struck a chord with me; it must be nearly 50 years ago I learned that acceleration is a change in velocity, and that this is obvious:

The correct mathematical answer is Yes. An object can be accelerating and not yet not moving.

I think some people here are getting velocity confused with movement.

georgeob1

2
Sat 23 Feb, 2013 07:33 pm
@dalehileman,
dalehileman wrote:

Sorry Eng, I'm in a state of virtual rigour
Rigor mortis perhaps.
dalehileman

1
Sun 24 Feb, 2013 10:29 am
@georgeob1,
Quote:
Rigor mortis perhaps.
Geo, more or less exactly
0 Replies

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