J.C.MacSwell

Since the water in the barrel will not rotate nearly as much as the ice we can ignore the angular velocity of the water completely and we have the following two statements of energy conservation:

 

[math]GPE_A = \frac{1}{2}Mv_A^2 + \frac{1}{2}I_{ice}\omega^2[/math]

and

[math]GPE_B = \frac{1}{2}Mv_B^2[/math]

 

Since the energy necessary to effect the rotation of the ice inside the barrel is nonzero, the linear velocity of the barrel must be less than that of the barrel containing water. Therefore the water-filled barrel will reach the ground first.

Agree under your assumptions, however, if the slope is long enough the increased friction from the water will slow the water barrel down and the ice barrel wins!

Janus is right. The vis-viva equation is

 

[math]v^2 = \mu\left(\frac 2 r - \frac 1 a\right)[/math]

 

The velocity of an object in a circular orbit at radius [math]a[/math] is thus

 

[math]v = \sqrt{\frac{\mu}{a}}[/math]

 

The delta-v needed to change a circular orbit at radius [math]a[/math] to an elliptical orbit with apogee [math]a[/math] and perigee [math]r[/math] is

 

[math]\Delta v = v\left(1-\sqrt{\frac{2r}{a+r}}\right)[/math]

 

For a geosynchronous satellite, a=42,164 km and v=3.0747 km/sec. You want to target an orbit with a perigee 60 km to 120 km above the surface of the Earth (R_E= 6378 km). Targeting a 60 km altitude requires a delta V of 1.492 km/sec; for a 120 km altitude this reduces to 1.486 km/sec. Both are 1.5 km/sec to two decimal places.

Thanks DH, now I see what Janus meant.

I was thinking a delta v around 2.5 km/s would be required to match the "speed" of the atmosphere at re-entry, getting it down around 0.5 km/sec and enabling a parachute to take over. This of course no longer being an orbit.

So in the above orbit, what is the speed at pergee/re-entry? Isn't it even higher than the original 3,0747 km/sec?

Hi.

 

As the shuttles, when returning to earth have to dissipate its enormous kinetic energy by counter propulsion (expensive) or braking trough the atmosphere; what if the orbit is geostationary ?

 

Would then a tiny impulse towards earth to start falling from orbit and then a parachute suffice ?

A satellite in geostationary orbit is moving quite fast relative to the Earth, any point on it, or it's atmosphere. (it has to, in order to "keep up" while following a much longer path)

The associated kinetic energy, along with the gravitational potential energy of it's height above the Earth, must be dealt with prior to any normal parachute being able to take over.

Merged post follows:

To go from geostationary orbit to one that would brush the atmosphere requires a delta v of about 1.5 km/sec. A tiny nudge would just put into an elliptical orbit with a slightly smaller period(it would no longer be geostationary) and with a perigee a little closer to Earth. It takes the 1.5 km/sec change to get that perigee to be within the Earth's atmosphere.

This seems low?

In decays (and collisions) the four-momentum is the thing to study.

 

If I recall rightly, in the rest frame you get conservation of mass.

I'm not sure what definition of mass would allow that. I think it would require photons to be considered to have mass.

Perhaps you are thinking of momentum?

Any chance the tygon could be absorbing water and expanding?

The glass of course would not do this and the geometry would stay consistent.

Do not listen to those people. Homoerectus is well documented. Whoever is questioning the validity of our past due to evolution is dead wrong.

Good Luck!

When I was a kid, grade 9, I had a teacher who claimed to be member of the Flat Earth Society. He had a number of arguments that made sense, in a superficial way. No one took him seriously, nor did he himself, but everyone enjoyed the debate, and there was no conflict with the particular course he was teaching, so I think he felt comfortable without letting on. He never admitted he knew better.

It has been claimed by many philosophers, theologians, and scientists that it is illogical to clain that anything infinite can be contained in something finite. For example, the human mind, something finite and imperfect, could never fully understand or comprehend or "become" God because God is infinite. Just like 5 L of water can't be contained in a 4L container, something of infinite volume cannot be contained in any finite container.

However, think about the number line. Between the 2 fixed points 0 and 1, there are an infinite number of points. This infinite amount of points is contained in a very finite space. If every point between 0 and 1 was a step, and you tried to walk from 0 to 1, you could never get there, regardless of the fact that the end does exist.

So, why can't this same idea be applied to the universe. Even if we consider that the universe is infinite in volume and matter, why can't we also think that it may be containable?

The part that really gets me is if the universe is contained in something larger than itself, what could it possibly be contained in. If we say that the universe is finite, then it follows that it must be contained in something larger than itself, but if we assume that it is infinite, then is it possible that it may be contained in something much smaller than itself, like the infinite amount of points be contained between 0 and 1?

I'm confusing myself, I better stop. But does anyone have any thoughts?

There are infinite points on a finite line, infinite lines on a finite plain, infinite plains in a finite volume, infinite volume in a finite 4 dimensional "space". etc

 

 

 

well, that makes things alot much clearer, though it doesn't answer the core question, the first time ever the selflessness gene was introduced, how did it spread from one element to another and a third till it had enough to compensate its self destruction?

 

if normal genes spread through groups at a linear rate by reproduction, and the selfless gene maintain and spread itself through a group exponentially through sacrificial behavior (just like y=x^2) then how did y reach the increasing interval, when it has to start from zero?(and pass through x<1, which is decreasing)

A genetic mutation can start from zero and multiply even if it has only negative effects, carried simply by chance and with support of all the positive genes. They are "all in the same boat" with each individual, the good genes and the bad, so there is a possibility for any gene to get established to some degree in a given population before it is really even tested.

You should take theory to mean an mathematical model or framework.

 

Think of string theory as a mathematical set-up that possibly could be used to construct realistic theories of nature.

It just seems to me it is more "string conjecture" at this point. In a scientific sense it really hasn't met the criteria that other scientific theories have, such as Relativity, Evolution, or even the Big Bang.

Why is String Theory considered a Theory in the scientific sense. Or is it?

Seems premature as it has not been solved/confirmed.

All my selfless acts were performed because they made me feel good.

Arguably they made you feel good because they were selfish.

What is it?

 

I don't understand.

 

If there is a bucket of water and you put a tube inside the bucket and blow at one end and then release water flows out of the tube. Why does this happen. Also when you raise this tube water stops. Why? This is not homework or anything.

Does this happen?

Imagine I have 3 bits of pipe connected in series A thin piece between two wide pieces.

If I force water through them the flow rate in ml/m must be very nearly the same because water is practically incompressible.

In the thin bit of pipe the flow velocity must be higher but the flow trates are the same througout.

With a given pump (or head of water) you would get more flow if you took out the thin bit and replaced it by (another) wide bit.

It's not velocity of blood that carries stuff about the body, it's the flow rate.

Everyone keep that in mind next time they wish to order a 3 egg cheese omelette.

Thank you both for help. So what is the pont of the bernoulli effect when it is not true. I mean when you constrict the pipe the molecules travel slower in real.

Constrictions generally add drag to the flow, but at the constriction itself the molecules must speed up for the flow to stay at the same rate. If the cross sectional area gets smaller, and the flow rate is the same, the velocity must have increased.

The Bernoulli effect is certainly true - as the velocity speeds up the pressure drops. It does this in a very specific way according to Bernoulli's equation, unless there are any significant shear effects, friction or turbulence, which must be accounted for.

Hello,

 

"Potential Energy (PE) + kinetic energy (KE) = Total Energy

 

When air travels through an airway at a CONSTANT FLOW RATE, the total energy of the fluid (PE + KE) decreases because friction converts some of this energy into heat. Since the fluid is moving at a constant flow rate, the loss is in potential energy (decrease in hydrostatic pressure)."

 

 

I don't understand this. Shouldn't the moving object lose K.E. Shouldn't resistance decrease speed.

 

I think the reason for that is I don't understand what is hydrostatic pressure, I researched but I don't get it.

 

"As we move down the airway, the tubes begin to narrow. When a tube narrows, fluid velocity increases causing an increase in KE at the expense of PE (ie. a compensatory decrease in hydrostatic pressure). This is referred to as the BERNOULLI EFFECT."

 

How does the fluid velocity increase when the resistance also increases when the tube narrows. Is the pressure inside this tube, exerted by moving molecules (KE) or hydrostatic pressure. I don't get it.

Those I bolded are very different concepts. Bernoulli's equations assume no friction, they assume an inviscid fluid, so they have to be modified in this case.

The velocity increases as the tube narrows due to the pressure gradient.

How do two black holes have their event horizons overlap, then "continue on their merry way"?

Imagine an ideal gas that is cooling down by radiation (it's probably in space somewhere, and it's warmer than it's surroundings).

 

The individual gas molecules have to emit a photon in order to cool down (probably, but not necessarily, in the infrared).

 

What happens at the moment that a photon is emitted? It must mean that the molecule is colder now. Temperature and velocity of a gas molecule are related (velocity is a function of the square root of the velocity). So, does the molecule instantly slow down?

 

And can this photon be emitted just in mid-flight of the molecule (without interaction with a second molecule), or only when it collides or interacts with another gas molecule? Does the emission of the photon have any influence on the trajectory of the molecule?

 

Some Friday afternoon questions from a chemical engineer.

It should be during the acceleration phase *. There must be a correlation between the acceleration and the wavelength. There must also be a momentum balance (as well as energy balance) so the photon would have to influence the trajectory of the interacting molecules.

Edit: * I just read Skeptics post, which makes sense, so the molecule itself need not necessarily be accelerating, but there should be an acceleration involved.

Thanks for the interesting replies. I would have thought someone here would mention special relativity which is what my mind snapped to when I first heard this question. I thought of the bucket in motion vs the water in motion. Isn't that...though infinitely small... a factor too?

 

Bee

I guess it would depend on whether you were measuring in "Bee time" or "bucket time".

He's probably referring to this:

http://en.wikipedia.org/wiki/Quantum_entanglement

Is there such a thing as an isothermic irreversible expansion of a gas against a constant external pressure? Could some one give me a physical example of one?

It can be done, at least approximately, slowly and carefully, at least if you accept a change of state as expansion.

That it's "irreversible" is a given for any heat transfer.

What are you trying to accomplish?

My pet theory is that the gum smackers distracted the non smackers and thus the whole thing was flawed from the start.

 

I'm not sure if I'm missing something but doesn't the fact that it's a gum manufacturer funding the science draw into question the science being done?

I think that should account for at least a -5% difference in test scores due to jealousy by the nonchewers alone.

Yet there was only 3% difference. I can only conclude that the gum chewing hurt the entire class, just more so in the control group of nonchewers.

I assume you mean the point of singularity. No it did not have a point in space, all space was existent within that point, space expanded with universe after the big bang.

I don't think it is currently accepted as being a point, or even finite. It may have been infinite in expanse, yet very, very dense, and expanded from that.

All that said, there is a symmetry apparent in the expansion, even in all directions, measured from points at rest with respect to the CMBR.

This symmetry changes when moving with respect to the CMBR.

"Something must know" is an example of the pathetic fallacy.

Yes it is. So...? (what does that have to do with what I posted?)

Did you get from my post some implication that space somehow "understood" what it is doing? I can't see why that should be necessary.

Space has properties. I don't know the mechanism (or whatever it is) involved but it has properties that it orchestrates extremely consistently.

To me that is not nothing. It's something.

More of it, for instance, and it takes longer to traverse than less of it. If it was truly nothing it wouldn't make any difference.

A person rotating in such a fashion would feel like they were standing in a gravitational field. Do you notice that you are standing in a gravitational field, rather than floating weightless? Given other equipment, there are other ways one could tell that one was rotating.

 

Your scenario is reminiscent of some ideas related to Mach's conjecture

http://en.wikipedia.org/wiki/Mach's_principle

I think that is rusty's point though (rusty, correct me if I am wrong)

While there is no non-relativistic ether, space is not "nothing".

Something (somehow) is doing the accounting.