so they don't fall in the hole

 

Next!

 

round is the only shape you can make it where the lid cannot be turned at any angle to fit through the hole itself, therefore protecting any men that have gone down into the hole.




I'm so slow

 

Probably because its weighs a ton and doesn't have sharp edges, safer if dropped

 

You have 9 marbles: 8 of them weigh 1 ounce each; 1 weighs 1.1 ounce. The 9 marbles are all uniform in size, appearance and shape. You have access to a balance scale containing 2 trays - you may use the balance 2 times. You must determine which of the 9 marbles is the heavier one using the balance only 2 times.

 

Googled it, I was wrong

 

this one is easier than the one in post #205:


You are given 5 bags. There are 10 beads in each of the bags. In four of the bags, the beads each weigh 10 kilograms. In the remaining bag, each bead weighs only 9 kilograms. All the bags and beads look identical. You must find out which bag has the lighter beads. The problem is that all the bags look identical and all the beads look identical. You can use a scale, but it has to be a single-tray scale, not a two-tray balance scale. Also, you may use the scale only once. How can you find out which bag has the lighter beads?

 

Would you even need a scale? That would be about a 20-pound difference between the lighter bag & one of the heavier ones - you could just feel it by picking them up, right?

 

Hmm yeah good point, I think that one got lost in translation somehow. The question I remember had the weights as 1 gram for the majority of the beads/marbles, and 1.1 for the other bag.

 

Ok... I thought it was a trick question. If it's grams, then... hell if I know!

 

Flawed example. A plane's jet engines don't drive the wheels.

 

Which ways more? 20 lbs of feathers or 20 lbs of lead?????????

 

wow this grew fast.

Eventually all of the engine power will be devoted to matching the resistance of the wheel bearings, leaving none to move the plane, and even if there was more, the treadmill would match it anyway.

Mike

 

Try again.

 

just when you think its done

 

What you guys are all forgetting is that this is not a frictionless environment.

It does take more force to move the plane when the wheels are spinning in the opposite direction. Put a toy with wheels on a treadmill(something heavy), attatch a pull scale. As the speed goes up the pull in the opposite direction will increase. Now, pull on the scale... Yes the plane will move forward, but it now takes the amount of force to hold the plane from flying backwards plus the force to pull the plane forward. Again, the faster it goes, the more that resistance factor goes up. So eventually, since the treadmill has an unlimeted top speed, it will overcome the power of the engines.

You are correct in saying it has nothing to do with the wheels creating propulsion, but there is a rearward pull.

Hers another example: Put a fan on the back of a toy car that is light enough to be blow forward. Now measure the speed of the toy car and place it on a treadmill going that speed, the power from the fan will now be devoted to overcoming the resistance from the wheel bearings, requiring the fan to be turned up to move the car forward. Turbines have a maximum RPM until they fail, whereas our hypothetical treadmill doesnt.

I win

Mike

 

There is no way the friction of the ball bearings and rubber in the airplane wheels is going to overpower the thrust from 2 or more jet engines.

 

So now you are placing bets... The amount of friction will go up until the bearings cook and sieze.

Mike

 

The problem is you are putting an unconstrained variable (the treadmills speed, and hence the force being put on the plane to go the other direction) against a contrained variable (the maximum output of the jets engines.

Lets have some math/aeronatical wiz calculate at what speed the treadmill would be moving for a 747 or any plane for the thrust of the engines to no longer be able to counter the force being applied by the engines. I believe it would be setup by setting the maximum power output of the engines equal to bearing resistance and the airliners weight along with tire stickiness(may be negligable).

And I couldn't agree more that this would be very interesting to see done on a smaller scale at M5 Industries.

Mike

 

No, you are not. The treadmills speed is not a force put on the plane in the other direction. The treadmill spins the wheels, which, other than a small amount of friction, have no affect on the movement of the plane. This friction could be countered with a fraction of the force exerted by the treadmill.

Last analogy...

You have a toy truck on a treadmill. There is a rope tied to this truck. You are holding the rope, standing in front of the treadmill.

When you pull that rope, does it make any difference how fast the treadmill is going? Would you have to pull 50 times harder if the treadmill was spinning at 50mph than if it was spinning at 1mph?

The answer is no, it wouldn't make any difference. The force of you pulling the rope is independent of the spinning treadmill and the spinning wheels of the truck. Just like the thrust of the jet engine is completely independent of the spinning treadmill on the runway.

 

As long as the pilot can maintain directional control, i think the plane would still take off even if the tires seized and they were burning rubber all the way down the belt.

 

Try this. Put a toy truck on the treadmill. Attatch the a rope to the truck over a pully and to a bucket. As the treadmill speed increases the truck will go back pulling the bucket up. Put some sand into the bucket, it will now require more speed for teh truck to pull the bucket up, but it will still do so. That sand is independant of the treadmill. The more sand added the more "thrust" used. at a certain point the bucket will become fill, and no more sand can be added, but as long as you can keep increasing the speed of the treadmill, the truck will continue to be able to be pushed back.

I am emailing Mythbusters BTW.

Mike

 

dude, a truck on a treadmill would not pull any harder as the speed of the treadmill went up. Perhaps a tiny bit from friction, but not enough to be significant.

 

Ok... Get a toy truck, fill it with 300 lbs of weights, attatch it to a string and to your nutsack. Turn on treadmill. When you get out of the hospital, tell me how the resistance is insignificant.

It is important to remember at speed all resistance is significant, especially when that speed is limitless.

You are at least admitting that there is some resistance... But you are stating it is negligable. .00000000001 *10^X can be significant if X is high enough. With a treadmill with no top speed, X = Infiniti. So the plane could never keep up, and hence, never take off.

Mike

 

Let me get this straight. Your argument is that at what...a million rpm?... the friction of the ball bearings in a wheel would be so great it would be stronger than the jet engines, thereby canceling out their force and stopping the plane from moving? And this would be caused by the extra rpms generated by the <200 mph speed the plane needs to take off?

Lets keep it within the bounds of reality.

 

You know how small this bearing friction is compared to the thrust of an airplane engine? Negligable doesn't begin to describe it. It's a non issue.

 

You know what I may be thinking...

That the treadmill is spinning fast enough to keep the jet in the same position at full thrust(Which is possible, on paper anyway) while you are simply stating it is spinning the exact speed the wheels are but in the opposite direction.

But then I'm still right because...

However, by doing that, it would increase wheel speed, and in turn have to increase its own speed, making its speed increase infinatly until the plane wasn't moving...

So I still win

Mike

 

Where did you get that equation for bearing friction? What is X?

Also, if you're point is that a small percentage of a large number is still a large number, I agree with that. However, we're talking about the vector sum of forces here. If the vector sum doesn't equal zero, then the plane will move.

 

Nope. The treadmill can only transfer force to the airplane through the bearing friction in the wheels. The bearing friction will never catch up to the thrust of the plane, because it doesn't keep increasing with increasing wheel RPM. In fact, the bearing friction will reach a maximum force that is negligable compared to the thrust force. As the bearign heat up, the bearing friction will actually become less and less - until the bearings melt, at whcih point it becomes zero.

 

The only true issue I could see if this were a real situation would probably be the tires exploding from the heat.

 

In which case the plane would still take off because the thrust of the engines would just drag the struts on the treadmill.

 

If a plane has full brakes on its not taking off... So the tires exploding and/or the bearings melting would more or less simulate that situation...

Mike

 

(a) why would melting be a simulation of brakes?

(b) if a plane has it's tire brakes on (as opposed to air brakes on the wings), full thrust of the engines would cause the tire brakes to fail. Easily.

 

 

(a) Melting bearing results in siezing. How To: Overtighten a wheel bearing on your car fill grease cover with sand.

(b) If you have a plane on a runway, you have the wheel brakes locked, and power up from there, the plane will not take off. I know this is definatly true with a prop driven plane, and I know that 747s are not supposed to remain at 100% Power for more than 20 seconds or the tarmack(sp?) starts coming up... I assume that means they are able to stay still.

Mike

 

The question centers around a force independent of the wheels being able to drive the plane forward on a perfect treadmill - The closest I've come to agreeing with Rootbeer is the argument that the wheels will just spin faster as the plane accelerates because the thrust is applied independent of the forces of the treadmill.

Try this experiment.

Attach your toy truck to a winch that will draw the truck across the floor at 5 mph (simulating thrust as the force moving the truck is independent of the wheels).
Now place this truck on a treadmill going 5 mph in the opposite direction and activate the winch. Does the truck move relative to the earth (ie, could it generate lift if it had wings?) I don't think so, I don't care what straight dope says, my non physics mind can't reason it out just yet. I'll start drinking and make a new formulation later.

Now, if that truck were already in the air, then it would move at 5 mph relative to the earth - I think.

 

It seems like the sum of the forces acting on the plane (treadmill vs forward thrust) is zero in a perfect system. Arguing about ball bearings or tires failing is missing the forest for the trees.

I am trying to reason out what would happen if the conveyor/treadmill were a passive system - ie free spinning but not actively being driven. I am inclined to believe that the plane would be able to fly in such a system.

 

(a) so it seizes...so what? does that mean that any amoun of force can't move it? Once you have wheel lock you can't move the vehicle no matter how much for you apply? Nope..all that will happen is the wheels will be dragged by the thrust....and the plane will still take off.

(b) How do you know this is definitely true for prop planes? and I have no idea what you mean by tarmac "coming up"

 

Most airplane wheels are passive. Planes cannot move unless their engines are producing thrust.

 

And by the way, our of curiosity, what are you basing any of your info on? Stuff you;ve actually learned? or just stuff you "think" is true?