trivia question of the day

Discussions are getting better !! still not there yet :lol:
Some of these are probably good questions for physics class
They are sometimes used in Mensa type questions

Sharyn
 
Wouldn't all the wheels involved have to be completely frictionless for the belt to move in sync. Does that have something to do with the answer. It's been too long since i took physics.
 
a sail boat on hydrofoils is place in a river with 5 knots of current on a day with ten knots of breeze.
assuming the boat hydroplanes when it reaches 4.5 knots and that normally the boat reaches 4.5 knots when the wind blows 15 knots.
is it faster for the boat to sail up stream or down? justifie your answer.

to see more about the sailing boat teh foiling moth... http://www.youtube.com/watch?v=lyAUc4128QA
 
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You can discount friction.

The answer is really simple but easily missed by most people
Hint what makes a plane fly, and how is that effected by the wheels and belt
Sharyn
 
Well, planes generate lift by the way air flows over their wings. So if the belt serves like a treadmill and the plane is essentially motionless it will never take off, no mater how fast the wheels and belt is moving.
 
This is an example of the wierd way things get classified
If you take the diameter of the head in sixteenths of an inch, take about one and double it you will have the classification
say the head is 6 sixteenths of an inch, that is a 10
Sharyn

Fascinating... wait, Ur, that can't be right. Consider a heavy hex verses normal hex, or a truss head verses fillister head machine screw, this much less a threaded rod.
 
think about it more carefully, most people will give your answer, but why is it not correct?
Sharyn
The plane would take off because it's the thrust propelling the plane, not the wheels. A dragster would stay in place because the wheels are pushing it forward. The plane's jet engines instead are the prime mover thus the wheels are just supports and going to spin at what ever speed they are moved at. IN other words, say double the speed tripple the speed etc until they wore out - that plane is still going forward because it's just a support.


Think a good headwind - draft is better thus actual takeoff speed can be less. The draft does the taking off not the speed the wheels are moving at. - good trick question.
 
Ship (Brian?)
of course is absolutely correct.
Most people make the mistake of thinking of a car, or walking where the power is applied via the wheels or legs to the surface. In a plane the power is applied to the plane, and the wheels really are only to reduce friction with the ground.

SO for instance if you stood on the brakes and the wheels could not turn then you would be testing the thrust of the engines vs the friction created by the brakes and the tires on the surface.

Sharyn
 
So who wants to go back and review the beam in the boat question and offer other explanations of their answer?

Sharyn
 
Very good question, sorry for jumping in, should have PM'd instead for a longer life to it. I admidt, and it is (Brian) that I innitially thought it was the reverse of this until I got the hints.

On the other hand, if there is enough thrust but not enough ability for forward movement sufficient to gain draft, the plane tends to follow gravity or direction of limited thrust - mostly the nose of the plane would tend to follow gravity. Really hard to compensate for thrust in the air however - that thing is going forward on it's base no matter if bolted to the floor or on a treadmill. Could be at an arc up or down but there is forward movement unless it really can't move.

This plane is taking off anyway no matter the speed of the wheels below it given in some way the wheels and treadmill were not sufficient to compensate for the forward thrust that is doing the propelling foward no matter how fast the wheels are going. Low air pressure and flying into a tail wind perhaps could be sufficient for this not taking off by way of a lack of forward progression. Such a case would still not be a question of the casters, but instead be a question still of the jet propelsion in the air verses that of the base the jet slides upon. The wheels, skids, pontoons or ice skis have limited effect.


A similar study might be all about air craft carriers in turning into or with the wind, and the slingshot aircraft launchers on some past ships that didn't even have runways.

Still good question and much to think about in understanding why. Heck for me, draft and the pressures involved with wing shape, much less propeller size shape and angle no matter if boat or plane are still just as much Voodo magik as DMX. I get the general point but don't mess with what I don't find the necessity to master.

On the boat, in general it should be boyancy and surface area but the weight of the beam with gravity will also tend to play a factor I would think. This again given hints as to it not being all that simple.

Used to watch on PBS a boat repair TV show often. Learned a lot about such things with that show and in general constantly found tech ideas. Also watched Junkyard Wars a lot and there was lots of science about propellers on it, much less on it and other shows the science about jets and wings for draft/low pressures. Learned more on TV than in science class.
 
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The key point in the boat question is

It takes more displacement of water to float an object since this is how it floats even though it is heavy steel boat etc.

So when the beam is in the boat, it is displacing more water (level is higher) because we have the weight of the boat and the weight of the beam. When we throw the beam out of the boat, the level drops and when the beam is in the water on the bottom the water displaced is just for the volume of the object not the extra to float it, SO the water level drops.

Most people think, you are throwing something into the water tank, so the water level will go up, but since you were supporting it by floating it via displacement it is just the opposite.

They are fun brain teasers where your initial thinking is typically false and the answer is counter intuitive.

Sharyn
 
Okay, I had to cheat and look up some first principles.

1. For something that floats, the weight of the floating object equals the weight of the water it displaces. (That was the one I didn’t know.)

2. For something that sinks, the volume of the object displaces the same volume of water. (Archimedes principle)

So with steel at 490 lb/cubic feet and water at 62.4 lb/cubic feet. Say the beam in the boat has a volume of 1 cubic foot so it weighs 490 lb. In the boat (see 1), it displaces 490 lb of water, or 7.85 cubic feet of water. So take it out of the boat, the boat rises, and the water level drops for the 7.85 cubic feet. Now drop the beam in the water, and it displaces 1 cubic foot (see 2) and raises the water level. The net change is a 6.85 cubic feet.


(I’m still having trouble grokking the plane problem.)

Joe
 
|Good job Joe, just to clarify it is a drop of 6.85.

Keep thinking on the Plane question ;-) That one usually starts a debate that goes on for ever !

Sharyn
 
The Plane problem:

Here’s where I’m having problems: One of Sharynf’s reply was “You can discount friction”. And I may be taking that out of context. Another of Sharynf’s replies was the example of using the brakes on a plane while firing the engines, and the friction of the tires on the runway counter the thrust, so the plane does not move. It seems there are two conflicting assumptions – discount friction or don’t.

And the original problem statement is perhaps ambiguous: Is it a non-powered belt (like a series of rollers) or a powered belt, like a gym treadmill? I think the original problem is that it’s a non-powered belt, but it says “…a rolling belt…” and “…the belt rolls exactly in sync…”. The latter implies (to me) a powered belt, like a treadmill or people mover. This makes it difficult to answer, but I don’t think it’s meant as a “trick”. [A classic “trick” question is two coins add up to 55 cents and one is not a nickel.] [Or maybe “belt rolls in sync” is a red herring and is to be interpreted otherwise.]

In order for this not to be a “trick” question, I’m going to consider friction. [If there is no friction, the tires simply do not grab, and the treadmill premise goes away. One might as well have a frictionless runway.] So, on a standard concrete runway, brakes on, engine fires: No movement. Next, on a standard runway, brakes off, engines fire, the plane moves, and wheels rotate in response to the friction between the runway and the tires.

Now, the rolling belt runway. First assume a non-powered belt. Now, brakes off and slowly fire up the engine. The plane is not anchored, so the wheel axles are not anchored, so the wheels begin to roll along the belt. The belt is free to move, but so is the plane. The thrust pushes the plane and the wheels roll along the belt, and the belt does not move. Presumably, the plane develops enough speed to take off. [I can’t convince myself about whether air resistance will cause some loss of thrust and hold the plane to some degree, allowing the belt to slip.] Thinking another way: set the brakes and fire up the engine, and the plane moves by causing the belt to roll.

Now if the powered belt is assumed (“…the belt rolls exactly in sync…”), how does the plane respond? (Never mind how the sensors work, assume they work.) If the plane starts to roll forward because of the friction between the tires and the belt, then the belt, by design, compensates and the plane does not move, by design. Or, consider this: put the plane on the powered belt, put on the plane’s brakes, and turn on the belt (on manual at a constant speed). The plane moves back. Turn on the plane’s engines and plane’s backward progress is slowed, until it finally overcomes the belt speed and moves forward. Switch the belt speed into “automatic” and the plane eventually becomes motionless.

[Am I missing something else in the premise? The whole concept sounds like some government pork barrel project….]


Joe
 
are the wheels and belt moving in the same direction? or opposite?
 
The wheels can roll forward and the belt moves forward also and is powered.
Bit the answer would be the same if the wheels moved forward and the belt moved backwards link on a teadmill

It is not really a trick question, but most people make a basic incorrect assumption that gives them the wrong answer.

Rolling friction of the wheels in not a factor, the tires do grip on the belt. The brakes are not locked, to the wheels are free to turn. The belt is powered so that it moves in the same direction as the wheels are rolling.
SHARYN
 
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the amount of water displace is realitive to the volume of the object, not teh mass, therefore a boat with big heavy thing in it would displace a lot of water, because it has a lot of surface area (if it were a smaller boat, it wouldn't float with the steel in it). by taking the beam out, the boat would rise, and the water would drop. the steel's would make the water level go up but not enough to compensate.
I really wish i had the time to cul through my phsyics notes and give you teh equation for all this, because i think i'd make more sense that way...
 
Ok, New Industry Related Trivia Question of the Day, < he said wrenching the steering wheel to the left so as to get back on track > :twisted:

What do you call a person who builds sets for movies, and why are they called that instead of what they really are ????

Cryptic enough for you ? anybody got the answer ??
 

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