will it take off?

[patrick_g]

Egon, depending on your definition of "FLOAT" or if "SUSPEND" on the surface is interchangeable. The pin, needle, whatever... can be suspended by surface tension such that it doesn't sink but some definitions of float refer to suspended by buoyancy due to water displacement. Some of both are happening but if your hands are not sufficiently rinsed from your last contact with soap/detergent you might find the trick difficult. If you do succeed, use a magnetized pin and get a compass.

Lets see now if I understand how posers are rated... If someone knows the answer cold then it is beneath their dignity and the offeror is a dolt but if an even simpler proposition is made that eludes a "TRUE" understanding by this critic then the offeror just doesn't get it. HMMMMMM

OK, Your server delivers your coffee at the begining of the meal but you want to drink it at the end. Not wanting to start a fuss with the server you decide to make the best of the situation that you can. Since you take cream in your coffee (the cream is essentially room temp) you have a degree of control. In order to have the highest possible temp of the coffee-cream mixture do you:

1. add and stir the cream immediately
2. carefully float the cream on the surface as a thermal blanket
3. wait till time to drink the coffee and then stir in the cream
4. add some cream now as a thermal blanket not stiring and add the rest later and then stir.

There are several other options that we clever sorts might envision from focusing the suns rays on the coffee with our reading glasses, covering the cup with a napkin, or starting a fire with several napkins but please choose from amomg the 4 above and defend your response.

This is an optional exercise. If it is too embarrassing to participate in such trivial frivolity or fraught with excessive danger of revealing your shortcomings, just skip it.

Pat

 

[RayH]

Add cream but do not stir.
By adding the cream you increase the mass of the coffee which will hold temp longer and avoid stirring because that, in effect, it increases the surface area and will dissipate heat faster.

 

[Egon]

Floating or suspending or lets say keeping the iron needle on top of the water in the glass is always easier if you handle the needle after having just finished wiping the excess grease from a hamburger off of your chin.

Not using cream in my coffee I have no empirical sensations to fall back on but I strongly believe ambient condition could indicate at least two answers to the ice-water-glass question. A third option may be possible.

 

[turnkey4099]

Floating or suspending or lets say keeping the iron needle on top of the water in the glass is always easier if you handle the needle after having just finished wiping the excess grease from a hamburger off of your chin.

Not using cream in my coffee I have no empirical sensations to fall back on but I strongly believe ambient condition could indicate at least two answers to the ice-water-glass question. A third option may be possible.

If the coffee is in a cup in the airplane, is it effected by inertia when the plane doesn't move?

Harry K

 

[daTeacha]

Presuming you are sufficiently skilled, adding the cream to just the top is probably the best solution. Minimize the amount of cream. Any stirring will transfer heat from the coffee to the spoon, cooling the coffee, unless you stir it a whole lot in which case the friction between the spoon and the coffee will begin to add heat to the coffee at the rate of 4.18 J of work raising the temperature of 1 gram of coffee 1 celsius degree, based on the false presumption that the specific heat of the coffee solution is the same or nearly the same as water. This solution also presupposes that some heat is transferred out of the coffee to the cup by conduction and from the cup to the air by radiation. If you are not skilled in adding the cream you will increase the amount of surface for this heat exchange to take place.

My turn -- Suppose you have a balance which will only work 3 times. You have 12 billiard balls which are identical in appearance except for an identifying number on each one. You are given the knowledge that one of the balls is slightly different in mass from the other eleven, but you do not know if it is greater or less. Your task is to find the odd ball and determine whether it has more or less mass than the others. You cannot determine any difference in mass between by any means other than using the balance and you can only use the balance 3 times. There are no tricks of any kind used in this, just simple logic.

 

[daTeacha]

I'll disagree on the aqueduct situation. If you have a pan of water and float a ball in the water while the pan is on a scale, you will see an increase in mass. The only way the boat would not add to the mass is if the displaced water is allowed to escape the aqueduct. Since there was no stipulation regarding whether the aqueduct was functioning in accordance with it's designed purpose of conveying water from point A to point B and thus had flowing water in it, nor whether it was brim full or not, one cannot provide a definitive answer. I don't think it will fly, however.

 

[daTeacha]

Simple one for some, not for others.

You are at a county fair. There is a vendor of homemade ice cream running his ice cream machines with a hit and miss engine. You stop to watch the engine run and begin conversing with the vendor. He offers you a free ice cream if you can tell him what the temperature of the ice cream is and how you know the answer. Since you really want some ice cream and have no money, what do you say? You do not have a thermometer available.

 

[daTeacha]

Once you get the needle floating on the water -- I mean a common sewing needle in a cup of water without artificially adding grease to your fingers and thence the needle -- how can you cause it to sink without substantially disturbing the water or cup? No magnets, please.

Oh yeah, I did the pool ball problem when I was a junior in high school for one lousy bonus point in algebra class. It took me one study hall and irritated the heck out of my teacher, himslef a pretty inventive guy who owned a bait shop. He converted a pop machine into a door that sold pre-packaged nightcrawlers and red worms 24 hours a day.

 

[Egon]

Pool balls, 12 of them; that is two more than Egon likes but Egon does like the number four!

 

[patrick_g]

If you read carefully my earlier comment regarding rinsing the soap/detergent off your hands, then the sinking of the needle wouldn't be a mystery! A bit of soap or detergent would reduce the surface tension which is helping to support the needle.

Some interesting thoughts about the coffe and cream from you guys but no onegot too close.

Adding the cream immediately lowers the delta T between the coffee and the ambient temp. The delta T is the big driver in heat transfer in this situation. Once the cream is added there is an immediate reduction of temp and the exponential rate of the coffee's approach to the asymptote (room temp) continues but the area under the curve (heat loss) is minimized. None of the other strategies are as effective. The main driver is delta T and the non-linear heat loss. Reducing the delta up front is THE TICKET.

You could argue the room is hotter than the coffee, you could spill it in your crotch and sue the place or any number of fanciful things but the best way to get thte warmest coffee for delalyed consumtion is to reduce the rate of heat loss by taking the "hit" up front when the greatest delta exists.

Teacha, before you disagree on the aqua duct you have to decide if the boat load of guys was in the water over the time of interest in the problem, i.e. not added by a crane or something midproblem.

If you go that route there is no change. If you assume the boatload of guys was added during the course of the problem, not just revealed, then of course the water level rises (Archimedes again) and the duct is stressed more.

Even at our best we often rush headlong into a "SIMPLE PROBLEM" without a good understanding of the initial conditions and then take up sides and argue about the stupidity of those who happened to see an assumption differently. Been there done that and have the scars!

Pat