The dumb questions thread

[steve_bank]

The problem was whether or not one understands Newtonian gravity on the surface of the planent. Loren saw it right off the bat.

First order approximations are useful, sometimes it is all you need.

Dumb question #2.

Adding real collisions to the problem how does that affect the answers?

Extra credit given for equations.

 

[Shadowy Man]

The problem was whether or not one understands Newtonian gravity on the surface of the planent. Loren saw it right off the bat.

First order approximations are useful, sometimes it is all you need.

Dumb question #2.

Adding real collisions to the problem how does that affect the answers?

Extra credit given for equations.

How does this qualify as a “dumb” question? Doesn’t seem fitting to the spirit of this thread.

 

[steve_bank]

The problem was whether or not one understands Newtonian gravity on the surface of the planent. Loren saw it right off the bat.

First order approximations are useful, sometimes it is all you need.

Dumb question #2.

Adding real collisions to the problem how does that affect the answers?

Extra credit given for equations.

How does this qualify as a “dumb” question? Doesn’t seem fitting to the spirit of this thread.

I was thinking of starting a 'dumb answer ' thread.

The thread has been a catch all thread rather than starting a new thread for each question. The dumb part of the op title is a bit tongue in cheek. Questions are more starting points.

 

[bilby]

The problem was whether or not one understands Newtonian gravity on the surface of the planent. Loren saw it right off the bat.

First order approximations are useful, sometimes it is all you need.

Dumb question #2.

Adding real collisions to the problem how does that affect the answers?

Extra credit given for equations.

How does this qualify as a “dumb” question? Doesn’t seem fitting to the spirit of this thread.

I was thinking of starting a 'dumb answer ' thread.

The thread has been a catch all thread rather than starting a new thread for each question. The dumb part of the op title is a bit tongue in cheek. Questions are more starting points.

Your opinion is noted, and rejected.

This thread has, from post one, been a place for questions that the poster feels they should know the answer to, but that they do not.

The place for questions that the questioner does know the answer to, and feels that others should too, is the classroom.

The members of this board are not your students, and your assumption that we can benefit from your pseudo-Socratic teachings is arrogant and insulting. We aren't here to solve physics problems for your amusement; We are here to help people who have genuine questions about things they feel dumb for not knowing.

If you are posting a question here for which you already have a solid understanding of the correct answer, you are doing it wrong.

 

[steve_bank]

The simple answer to the last question is Newtonian mechanics.

Loss of energy reduces momentum which reduces speed. Unless one wants to nitpick over the mass of the ball changing by giving up some mass by friction in the collision.

Time to hit the ground remains the same, number of transits may go down, and the spot where it hits the ground changes.

 

[Swammerdami]

The simple answer to the last question is Newtonian mechanics.

Loss of energy reduces momentum which reduces speed. Unless one wants to nitpick over the mass of the ball changing by giving up some mass by friction in the collision.

Time to hit the ground remains the same, number of transits may go down, and the spot where it hits the ground changes.

"Which reduces speed" and "Time to ground remains same" contradict each other. Assuming inelastic collisions, the first is correct, the second not.

Also the ball will be acquiring and imparting spin, which will change the collisions. (Details beyond my pay grade!)

So I think for such a question to both have a definite quantitative answer and be "dumb," perfectly elastic collisions need to be assumed. In that elastic vein, let me recommend #876 ("a way to derive the digits of pi from a ball-bouncing experiment") to your attention. In addition to the link I gave, there's a YouTube and a math journal article devoted to this remarkable tidbit.

 

[Bomb#20]

There are two perfectly parallel vertical plates separated by 1 meter. Initial conditions a small ball at 10 meters high is starting away from one plate directly at the other plate.

The problem was whether or not one understands Newtonian gravity on the surface of the planent.

But your scenario can't be on the surface of a planet -- it can only exist in outer space in an accelerating rocket. On the surface of a planet if the plates are both vertical then they aren't perfectly parallel -- the tops are necessarily a couple microns further apart than the bottoms. It's not like your planet can have infinite diameter.

 

[steve_bank]

There are two perfectly parallel vertical plates separated by 1 meter. Initial conditions a small ball at 10 meters high is starting away from one plate directly at the other plate.

The problem was whether or not one understands Newtonian gravity on the surface of the planent.

But your scenario can't be on the surface of a planet -- it can only exist in outer space in an accelerating rocket. On the surface of a planet if the plates are both vertical then they aren't perfectly parallel -- the tops are necessarily a couple microns further apart than the bottoms. It's not like your planet can have infinite diameter.

It is just a simple thought experiment. Don't over think it.

If you wnt to flsh it it out gravity is not excatly constant on the surface changes with altitude. A ball s mot perfectly spherical.

To calculate a high order solution beyond an approximation would require a simulation. It is beyon me, the actual deformations of te ball and plate can be simulated.

In pool with stiff balls an elastic collision assumption works well. The same with many practical situations.

https://en.wikipedia.org/wiki/Elastic_collision
https://en.wikipedia.org/wiki/Inelastic_collision

 

[Jokodo]

So here is my dumb question:

According to everything I find on the webs, the caloric value of sugar is roughly 400 kcal/ 100g, while for alcohol (ethanol) I find 700kcal/100g.

What's confusing me is that ethanol is the product of fermenting sugars in what I must assume is an exothermic reaction. I mean, it's what yeasts live of, right? I'm aware that this reaction produces carbon dioxide as a side product, but I still can't seem to get the math to work. 1 mol of glucose (180g) produces 2 mol of carbon dioxide (44g each) and 2 mol of ethanol (46g each), so the resulting 82 grams of ethanol must have very nearly the same caloric value as the original glucose (692 for 100g of ethanol vs 757 for the sugar required to obtain those 100g if I plug in concrete figures I find). Is alcoholic fermentation really that marginally exothermic?

 

[bilby]

So here is my dumb question:

According to everything I find on the webs, the caloric value of sugar is roughly 400 kcal/ 100g, while for alcohol (ethanol) I find 700kcal/100g.

What's confusing me is that ethanol is the product of fermenting sugars in what I must assume is an exothermic reaction. I mean, it's what yeasts live of, right? I'm aware that this reaction produces carbon dioxide as a side product, but I still can't seem to get the math to work. 1 mol of glucose (180g) produces 2 mol of carbon dioxide (44g each) and 2 mol of ethanol (46g each), so the resulting 82 grams of ethanol must have very nearly the same caloric value as the original glucose (692 for 100g of ethanol vs 757 for the sugar required to obtain those 100g if I plug in concrete figures I find). Is alcoholic fermentation really that marginally exothermic?

Yes, it is. But a little is infinitely better than nothing, and when you have no source of additional oxygen, you have to make do with what you've got.

That's why yeast goes with aerobic respiration rather than anaerobic fermentation, if you don't restrict its access to oxygen.

 

[Jokodo]

So here is my dumb question:

According to everything I find on the webs, the caloric value of sugar is roughly 400 kcal/ 100g, while for alcohol (ethanol) I find 700kcal/100g.

What's confusing me is that ethanol is the product of fermenting sugars in what I must assume is an exothermic reaction. I mean, it's what yeasts live of, right? I'm aware that this reaction produces carbon dioxide as a side product, but I still can't seem to get the math to work. 1 mol of glucose (180g) produces 2 mol of carbon dioxide (44g each) and 2 mol of ethanol (46g each), so the resulting 82 grams of ethanol must have very nearly the same caloric value as the original glucose (692 for 100g of ethanol vs 757 for the sugar required to obtain those 100g if I plug in concrete figures I find). Is alcoholic fermentation really that marginally exothermic?

Yes, it is. But a little is infinitely better than nothing, and when you have no source of additional oxygen, you have to make do with what you've got.

That's why yeast goes with aerobic respiration rather than anaerobic fermentation, if you don't restrict its access to oxygen.

Thanks!

Follow up question: The further fermentation of alcohol to acetic acid is aerobic and yields much more energy, but still less than a full respiration of the sugar/alcohol to CO2 and water. So in the event that vinegar is what you want to get, rather than what you will get if you don't sufficiently restrict oxygen access, what do you need to do to prevent the germs from going all the way and ending up with carbonated reduced-sugar juice? Is it just selecting strains that don't have the full metabolic pathway, and betting on them being fast enough to do their thing that the resulting liquid becomes to acidic for other strains before those can take over?

 

[TomC]

what do you need to do to prevent the germs from going all the way

Germs?
Germs!?

Those little guys are turning water into wine! Which you cannot do by yourself.

Micro-Gods, for your information.

Hmmph...
The nerve of some people.
Tom

 

[lpetrich]

Let's look at the chemical formulas. Sugar monomers like glucose and fructose have formulas C6H12O6, while drink alcohol (ethanol) has chemical formula CH3-CH2OH or C2H6O -- much less oxygen. Table sugar (sucrose) is glucose + fructose - water.

Sugar monomer C6H12O6 -> 2 * lactic acid: CH3-CHOH-COOH or C3H6O3

Lactic acid -> ethanol + CO2

So sugar -> alcohol (less oxygenated) + CO2 (more oxygenated)

 

[steve_bank]

Praise be the holy bacteria, creator of the sacred stupor.

 

[Jokodo]

what do you need to do to prevent the germs from going all the way

Germs?
Germs!?

Those little guys are turning water into wine! Which you cannot do by yourself.

Micro-Gods, for your information.

Hmmph...
The nerve of some people.
Tom

Oh, but I was talking about the ones that turn eine into vinegar in the passage you quote...

 

[lpetrich]

Praise be the holy bacteria, creator of the sacred stupor.

Strictly speaking, the holy yeast. It's a eukaryote, not a prokaryote, like a bacterium, but it does live in very bacterium-like fashion.

 

[steve_bank]

Praise be the holy bacteria, creator of the sacred stupor.

Strictly speaking, the holy yeast. It's a eukaryote, not a prokaryote, like a bacterium, but it does live in very bacterium-like fashion.

Learn something new everyday.

 

[bilby]

Praise be the holy bacteria, creator of the sacred stupor.

Strictly speaking, the holy yeast. It's a eukaryote, not a prokaryote, like a bacterium, but it does live in very bacterium-like fashion.

The Fermenter, the Scum, and the Holy Yeast...

 

[Wiploc]

Here is a dumb question. Is the universe rotating?

I'll say no. If it were rotating, it would have a center.

 

[Wiploc]

Here is a dumb question. Is the universe rotating?

Compared to what?

I'll say no. If it were rotating, it would have a center.