Split Thread The validity of classical physics (split from: DWFTTW)

[Dan O.]

Why do people keep describing it as "accelerating" due to "gravity" at g m/s/s? Why was it all right to discuss objects accelerating towards terminal velocity before, and no-one said "No, they're not accelerating."

John, it's quite simple. Sometimes we don't use an inertial reference frame to discuss motion. Since we live in an accelerating world, we tend to use a reference frame that is also being accelerated at 9.8 m s^-2 away from the earths center. From our accelerating reference frame, we describe the motion of a ball tossed into the air as being accelerated by gravity instead of the reality that we are being accelerated towards the free floating ball by the repulsive force of the ground we stand on.

Accelerometers are generally ignorant of our chosen accelerating reference frame so they measure acceleration with regard to their own inertial reference frame and therefore claim that they are being accelerated when at a fixed point on the surface of the earth.

 

[Michael C]

I just made a couple of drawings to try and show the distinction between what accelerometers see if the cart is sitting on the ground and experiencing 1g vs. a cart being accelerated in the absence of a gravitational field. I imagine everyone already gets this, but I was in the mood to make the diagrams.

Careful! The arrows in your second diagram need to be pointing up, not down. An accelerometer held stationary at the surface of the earth reads 1 g vertically away from the earth. That is the answer to the bonus question.

 

[humber]

Humb is far, far superior to Humber. Humber occasionally and inadvertently mixes facts into his stories, thus causing irrevocable internal inconsistency and therefore horrible damage to his arguments. Humb on the other hand has a clarity of nonsense, which is pure enough to stand unbreached.

Humb doesn't paste you, that is the difference.

 

[humber]

You clearly make things up just for effect. I don't think humb would even say this to make fun of you.

Then tell him not to say it.
Of course, your entire claim of wind speed and the means by which things gets there, resulting in zero force, is based entirely upon the notion that the force derived from the wind, falls with the velocity of that object. If fact, a parachute has been given many times in support of that claim. The treadmill disparately requires that this be limear with velocity. You do have a short memory.

No you won't.

I will get back to it, but it's more fun getting at you.

John, if we model the Earth as a true sphere we can think of it as an infinite number of infinitely thin shells. When we are outside the Earth we can model the whole thing as being the total mass of the shells at the very center. When we are inside the Earth something whacky happens... When you are inside a homogeneous spherical shell you feel no gravitational pull from that shell - no matter where in the shell you are.

Not for a body of any size. There is a gradient, a consequence of those "shells." This also applies for macroscopic objects is an electrical potential field. Same thing.

So, whatever your depth beneath the ground, you have to ignore the contribution of all of the shells whose radius is greater than your distance from the center of the Earth. As you get closer to the center there is therefore less Earth mass effectively acting on you. And this reduction in mass happens more rapidly (cubed law) than the increase in gravity due to your closer proximity to the center (squared law).

Gravity is not local to that extent. All that could possibly mean, it that the force upon the object is equal on each direction.

Also, what Sol said about gravity and acceleration is on the money (of course). Basically, you can consider yourself to be in an inertial frame if:

A) You're free-falling in a gravitational field (and therefore accelerating!) - in which case you will ignore the effect of gravity entirely.

Absurd. There is a distinction between gravitational force, and acceleration due to that force.

OR

B) if you are not accelerating - and then you must consider the force of gravity as an external force.

Oh goody, there's a choice.

No, you must wonder why you are not accelerating and then look for the culprit. Perhaps air resistance.

Gravity and acceleration are wierd that way. Of course as Sol points out, the only distinction that can be made between gravity and linear acceleration are the gradient effects (lines of gravity are not parallel, but converge toward a point - and gravity is stronger toward the source).

wierd* A standard scientific term.

No, I am afraid not. The Earth's gravitational field is not homogeneous. There is a gradient of course, because force is related to the square of the distance. For objects close to the surface, their altitude is likely to be small compared to the diameter of the Earth, so the change is small.
There are local differences and g varies across the planet. (Some weighing scales must be calibrated to account for this. It is not enough to use a known mass, because that is also effected.)

Seems much ado about nothing. If the gravitational force is uniform, then it will look like a constant force, and so accelerate the object accordingly.

 

[humber]

So a chute moving through air is not the same as a chute moving in wind. Interesting.

The force driving the chute come from the car in your case. When released, it will move through the air at the speed of the car. The car will continue to drag the chute, until all of it's KE is dissipated as friction. That's how they work.
A chute pulling a load is otherwise. The wind imparts momentum to the chute, but as it's velocity increases, the differential velocity between chute and wind falls, so less momentum may be transferred. Thus, the force falls with chute speed. However, it's not all bad, because maximum power output is achieved before then.

Or did you change your mind? Are you agreeing that the general equation for drag applies to a parachute as it is moving through air or in wind?

No, they are what is known as "different", Mender.
Moving, wind-borne chutes do have drag, but there's more to it than that.

 

[humber]

Dear gravity, please stop acting in proportion to the mass of the two bodies and inversely as the square of the distance between them.

Dear fairies, make my treadmill work.

 

[humber]

I like it. The statement is actually true for a certain interpretation with humber's chosen frame of reference^*but in general it is not.
^* Brought to you by the letter L

• "For a such harmonic motion dv/dt is greatest at zero crossing." #3081

No Mender. It is simply correct.
Don't bother submitting it though, as it will be buried in the same way you see cats so often do.
Still, some answers to those already posted my be useful, though some cats only like soft foods.

 

[RossFW]

Wow!!

I think we've reached a water-shed now. We are having an informative and entertaining discussion of Physics, whilst simply ignoring Humbers nonsense!

Let's keep that going.

Dan,

I can model what you are saying, and in fact said as much to John. That being said, I share one point of confusion with him. Whilst I understand(and have experienced!) the fact that being in freefall is the same as being under no gravitational force, and that there is no way to measure acceleration in that state, you ARE accelerating from the frame of reference of the surface of the body supplying the gravity. How does the fact that in one case you have no acceleration, whilst in the other you do, square with acceleration being absolute?

While writing it occurred to me that, I suppose,from the frame of the surface the object is accelerating and therefore CANNOT be thought of as inertial frame, and from the frame of the object, the same can be said of the surface,.

Am I on the right track?

 

[spork]

Ummmmm... check your PMs, spork.

D'OH!!!

humb pointed out that the pointy things are going the wrong way in one of my diagrams. Here are my excuses:

A) It was early - O.K.!?
B) It was intentional - I was conducting a "spot the flaw" exercise
C) The sun was in my eyes
D) I tripped on a rock.

In any event, I got it wrong (think about that concept humber).

Updated figures below.

 

[Michael C]

Spork, you corrected the diagram but not the explanation underneath!

 

[Dan O.]

Here is a twofer nomination for the Where humber is wrong list:

• "Hot air balloons raise their potential energy in the gravitational field by gaining altitude. This energy can be used to gain velocity, leading some to think that the balloon is traveling faster than the wind." #147
  
  
• "A balloon can raise its altitude, and gain potential energy. When descending, that energy is converted to lateral velocity. " #447
  

Rather than ballooning the list with duplicate entries, I think I should just keep one and tag it with both references.

 

[humber]

John,
Have a look at my zero-g training video. These people are REALLY in true freefall. The aircraft can be flown to exactley counteract forces due to air resistence, so that all that is left is gravity, exactley as if in a vacum.

That's almost correct !

I've done this many times in Aerobatic aircraft. What's on the panel is an accelerometer and the whole point is to go fron +1 to 0 'G'.It's FUN!

As I said (and Humber couldn't understand) it is indistinguishable from being in zero G in space.

No. Gravity is not "canceled out", but the consequences of the external environment removed. That is not the same as zero g.
A very common misconception, repeated by you, Captain.
Like "vomit comet"; something for the tourists.

As someone else said, you are ALWAYS being accelerated by gravity anywhere in the universe, but as everything in YOUR frame of reference is in uniform acceleration, it is exactley the same as a complete absence of gravity.

OK, let's get rid of gravity, shall we?

ONLY if something stopped it moving towards that body, and so produced a reactive force.

The accelerometer would also see the result of that force.

If the accelerometer waswere in freefall, it would simply start a uniform acceleration towards the body, and still read zero.

If it's accelerating, the output of the accelerometer produces an output proportional to that acceleration. It does not stop doing so if the acceleration is "constant". Acceleration is change in velocity w.r.t time.
Accelerometers are expensive devices (often inversely proportional to size). If they were to operate as you suggest, then I would want my money back.

In it's simplest form, an accelerometer is a suspended weight that registers acceleration due to it's own inertia relative to the rest of the instrument. If the WHOLE INSTRUMENT, INCLUDING the weight, is subjected to a uniform acceleration, the weight stays where it is relative to the rest of the instrument, and thus registers zero G

Oh dear. A simple accelerometer may consist of a mass on the end of a small spring or lever. Strain gauges or piezoelectric sensors are often used to measure the effect upon that lever, as the mass attached to it is accelerated. That is "the whole instrument".

The mechanical compliance of the the spring or lever, decouples the sensing mass from the instrument's body mass.
Output may be produced by merely swinging an accelerometer.

You do not seem to understand your instruments, RossFW; something alarmingly common amongst pilots.
In one case, the ground crew were blamed for not removing tape placed over a pilot tube, put there as part of the cleaning process.
Because the Captain did not understand his instruments - those he is trained and paid to understand - he trusted them beyond all reason, and crashed the plane.

 

[Dan O.]

If you hollow out a large depression in a planetary body, can you create a situation where the gravitational field gradient is indistinguishable from that of acceleration over some bounded region such as inside an elevator car?

 

[RossFW]

If it's accelerating, the output of the accelerometer produces an output proportional to that acceleration. It does not stop doing so if the acceleration is "constant". Acceleration is change in velocity w.r.t time.
Accelerometers are expensive devices (often inversely proportional to size). If they were to operate as you suggest, then I would want my money back.

Then get onto paypal, Dude. I had a state-of-the-art one in my Extra 300. When I accelerated towards the earth at 9.8m/s^2 it read ZERO!!

Humber, if the entire instrument increases it's velocity at a uniform rate (as it does under acceleration due to gravity) it won't sense the mass being accelerated, as all the SENSORS are being accelerated at the same rate. Ther IS no strain on the member, as every atom in it is accelerating in the same direction at the same rate.

It's the WHOLE relative thing that you just don't get. If I try to judge my acceleration by how fast the person sitting next to my dissappears in the distance, but he doesn't because HE'S accelerating at the same rate, I will measure my acceleration as zero.

 

[spork]

Spork, you corrected the diagram but not the explanation underneath!

I have no idea what you're talking about. The explanations are perfect.

(of course I shamelessly went back and edited them now that you've pointed it out )

 

[humber]

Here is a twofer nomination for the Where humber is wrong list:

• "Hot air balloons raise their potential energy in the gravitational field by gaining altitude. This energy can be used to gain velocity, leading some to think that the balloon is traveling faster than the wind." #147
  
  What a clanger. Only an idiot would argue that an object cannot gain lateral velocity from potential energy gained from a gravitational field.
  Do the words 'tracrix' or 'catenary' mean anything to you?
  
  I saw a hot air balloon just last night. Still wind to me, but I could see from the burner that it was moving quite fast.
  Also, do you think that the expelled hot air may do something?
  I think I will award that one 4 'spilled milk' points.
  
  
• "A balloon can raise its altitude, and gain potential energy. When descending, that energy is converted to lateral velocity. " #447

Rather than ballooning the list with duplicate entries, I think I should just keep one and tag it with both references.

I would bury them in the same spot in your litter tray.

 

[spork]

If you hollow out a large depression in a planetary body, can you create a situation where the gravitational field gradient is indistinguishable from that of acceleration over some bounded region such as inside an elevator car?

I'm almost certain that you can't. But with my recent history of getting simple things wrong you should almost certainly get a second opinion.

It seems to me that if you could to this, it would be over a bounded region that had no dimension - but that takes us back to the fact that gravity and acceleration are already indistinguishable in that zero-dimensional region.

But it's a curious question. I'll (probably) try and think of a way to attempt it.

 

[RossFW]

What a clanger. Only an idiot would argue that an object cannot gain lateral velocity from potential energy gained from a gravitational field.
Do the words 'tracrix' or 'catenary' mean anything to you?

I saw a hot air balloon just last night. Still wind to me, but I could see from the burner that it was moving quite fast.
Also, do you think that the expelled hot air may do something?
I think I will award that one 4 'spilled milk' points.

Honestly. Humber. If you live near a ballooning site, for chrissake go and ASK a balloon pilot if what you say is right!

$1000 bucks US says you aren't!

 

[humber]

D'OH!!!
humb pointed out that the pointy things are going the wrong way in one of my diagrams. Here are my excuses:

As if we need to be told. Of course there would be PM's away from prying eyes..

So let's see:
A) It was early - O.K.!? (Sorry, I came to soon)
B) It was intentional - I was conducting a "spot the flaw" exercise (Wily Coyote)
C) The sun was in my eyes (I didn't see my own trap of above)
D) I tripped on a rock. (I keep banging then together)

E)All of the above, to avoid simply saying that you made a mistake.

In any event, I got it wrong (think about that concept humber).
Updated figures below.

When you finally get your crayons out, you do make a mess.

As Micheal_C points out, it still makes no sense. If all objects within the elevator are accelerated the same amount, and that is the same as the elevator, I would expect that everything would 'stay in place'. However, the accelerometers will measure the acceleration.
Now that's a problem. How do you do that, without at least one moving part, that is not quite moving like all the other ducks in a row?

The second drawing appears to suggest that if objects are in a gravitational field, but moving, acceleration may be detected? I presume these are 3-axis accelerometers?

Phone NASA, no the Whitehouse, now.

 

[Michael C]

As Micheal_C points out, it still makes no sense.

No, that's not what I pointed out. I pointed out that Spork had forgotten to change the explanation to match the diagram. Now he's done so: the diagrams and the explanations below them now make complete sense.