Wikipedia:Reference desk/Archives/Science/2017 August 26

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 26

Walden Farms

What exactly is in Walden Farms products that makes a whole jar of pancake syrup have zero calories? — Preceding unsigned comment added by 82.43.219.147 (talk) 08:48, 26 August 2017 (UTC)

Ingredients lists for all their products are available here. Rojomoke (talk) 10:07, 26 August 2017 (UTC)

Note that their "SAVE 10,000 CALORIES A MONTH" claim is highly suspect. According to some studies, using artificial sweeteners does not seem to actually cause weight loss. The reasons are unclear, but one possibility is that the perceived sweetness causes the release of insulin, which causes blood sugar to plummet, making you crave sweets. The other possibility is just that your body expects a certain number of calories per day (or specific constituents like protein, fats, and carbs), and if it doesn't get them from one source, it makes you feel like you are starving to death until you get them from another source. A more honest claim might be "EAT AN ADDITIONAL 10,000 CALORIES IN OTHER FOODS EACH MONTH !". StuRat (talk) 17:35, 26 August 2017 (UTC)

• Nothing with any calories.

Enough of a non-calorific gum to make some sort of viscous liquid or colloid, when mixed with water

Non-calorific sweeteners, such as sucralose

Concentrated flavourings

It's more of a food pill than a foodstuff, but it's calorie free and free from "those evil carbs". Andy Dingley (talk) 12:47, 26 August 2017 (UTC)

Note that the nutrition information these doesn't give an amount for even 100g, let alone a whole jar. It only gives an amount per serving and serving sizes are normally either 1/6 or 1/12 jar of a jar. So the zero calories only means it does not meet the minimum to have any calories in one serving. In any case since the syrups all seem to be flavourings, water, preservatives, ingridients to give the desired consistency and acidity and sucralose as the sweetener you should not expect many calories. Nil Einne (talk) 12:46, 26 August 2017 (UTC)

Feynman Lectures. Exercises. Exercise 12-5 JPG. Lecture 12

. .

...

12-5.* A side view of a simplified form of vertical latch is as shown. The lower member A can be pushed forward in its horizontal channel. The sides of the channels are smooth, but at the interfaces of A and B, which are at 45° with the horizontal, there exists a static coefficient of friction μ. What is the minimum force F that must be applied horizontally to A to start motion of the latch, if B has a mass m?

—  R. B. Leighton , Feynman Lectures on Physics. Exercises

I have solved the exercise png, but I've found that if
$mg{\tfrac {1-\mu }{1+\mu }}<F<mg{\tfrac {1+\mu }{1-\mu }}$, then
$\mu \neq {\text{const}}$, with $\mu =0$ at F = mg.
How is it possible?
Username160611000000 (talk) 11:57, 26 August 2017 (UTC)

It seems that when motion doesn't begin the friction force rises until breakaway. Username160611000000 (talk) 12:52, 26 August 2017 (UTC)

That looks like a pretty complicated situation to me, and I'm not confident of an answer yet. The smooth walls are applying some kind of torque to the vertical element, with multiple forces to compensate for the tendency of the horizontal element to turn it. But what strikes me most is that if the joint is freely moving, the rear wall applies relatively little pressure -- but if it is stuck, then when you push in the latch, you must be putting considerable force on the rear wall. Does that make any sense? Wnt (talk) 15:42, 26 August 2017 (UTC)

See the Wikipedia article Wedge that describes the simple machine involved. B will move at the same speed as A so the mechanical advantage is unity (1). With no friction the force F is just the weight of A which is mg. The source of friction is the force normal to the surfaces in contact which is mg/√2. The force required along (i.e. in the slanting direction of) the surfaces to start them sliding is μmg/√2. The otherwise frictionless arrangement gives the pusher wedge no mechanical advantage in applying this force either. So my result is (1+μ)mg/√2. Blooteuth (talk) 22:14, 26 August 2017 (UTC)

is the force normal to the surfaces in contact which is mg/√2 -- $N\neq {\tfrac {mg}{\sqrt {2}}}{\text{ , but }}N=mg{\sqrt {2}}$. You can check that if you consider all forces, applied to member B. I have understood that magnitude of friction force varies like this . In ex. 12-5 as F grows, such a moment (namely F = mg) is possible when there is no parallel force. Feynman didn't say that, but correct formula for friction force is $F_{\text{friction}}\leq \mu N$. Username160611000000 (talk) 04:38, 27 August 2017 (UTC)

You can consider the balance of mechanical work when A moves by small distance $\delta x$. The work of the force ($F$) applied to A is $\delta xF$. Due to the geometry B should move by the same amount - $\delta x$ - up, so work against the gravitational force is $\delta xmg$. On the other hand the surfaces of A and B will move ${\sqrt {2}}\delta x$ relative to each other. So, the work against the friction force ($F_{f}$) is ${\sqrt {2}}\delta xF_{f}$. The friction force itself is proportional to the force that presses A against B or

$F_{f}=\mu \left({\frac {mg}{\sqrt {2}}}+{\frac {F}{\sqrt {2}}}\right)$.

You should note that the pressure force is a sum of two terms (Blooteuth missed the second term). So we have the balance:

$\delta xF=\delta xmg+{\sqrt {2}}\delta x\mu \left({\frac {mg}{\sqrt {2}}}+{\frac {F}{\sqrt {2}}}\right)$.

The final result is

$F=mg{\frac {1+\mu }{1-\mu }}$.

So, no motion will happen if $\mu \geq 1$. Ruslik_Zero 18:47, 27 August 2017 (UTC)

Perception through yellow sunglasses

Yellow tinted sunglasses give the impression that there's more light (and that there's more contrast). Indeed, since they are filtering blue light, there is less light. What makes us feel there is more light and what makes us see more contrast? --Hofhof (talk) 16:24, 26 August 2017 (UTC)

Part of it is likely that the iris contracts or expands based on the light level detected. So, if less light gets past the sunglasses, the iris may expand to adjust for that. However, there seems to be another part, that blues and purples are seen as "dark" colors, perhaps because our cones and rods are less sensitive to them than "bright" colors. It would be interesting to know exactly what controls the iris/pupil diameter, and if that is highly sensitive to blues and purples. StuRat (talk) 17:26, 26 August 2017 (UTC)

Purkinje effect, Purkinje effect#Use of red lights, Adaptation (eye) Night vision#Biological night vision, Accelerating dark adaptation in humans. --Guy Macon (talk) 20:53, 26 August 2017 (UTC)

Monday morning, while waiting for darkness, I looked at the ground and was puzzled. The dead grass and the soil looked normal in color, and yet I found myself thinking that they ought not to be so yellow! Later I thought: when ambient light is so dim, I expect it to be blue, the color of twilight.  Maybe the Purkinje effect is related / was involved too? —Tamfang (talk) 05:52, 27 August 2017 (UTC)