Anybody up for a riddle?

Boydster bought this one
That was very well done son!


Anybody up for another?
Please post one for fun or for your mother
I've been applying for jobs all day
And my brain is now dead I tell you stingray
I think that's enough said.
But too early for me to head off to bed.

This is an old favorite. You are given two metal cylinders identical in size, shape and weight and told that one is made of magnesium and one of osmium. They are covered in a tough epoxy paint so you can't see the metals, can't poke with a knife to see which is softer and can't bash with a sledgehammer to see which breaks (well maybe you could but we'll tell you that that is not the answer). What very simple test(s) can you do (no instruments) to tell which is which?

ajdelange said:

This is an old favorite. You are given two metal cylinders identical in size, shape and weight and told that one is made of magnesium and one of osmium. They are covered in a tough epoxy paint so you can't see the metals, can't poke with a knife to see which is softer and can't bash with a sledgehammer to see which breaks (well maybe you could but we'll tell you that that is not the answer). What very simple test(s) can you do (no instruments) to tell which is which?

Click to expand...

This is only a guess
But I'd put a magnet to the test
One might look like the other
But if my thinkings right they aren't exactly twin brothers
Ones missing some trons I don't know just which
My answer is just me being a ... hmmm.. nothing rhymed with witch

They doubtless have different magnetic properties and I can think of tests based on that. A magnet is a more sophisticated instrument than is required here.

Aj Delange. Im certainly not clear the answer to the riddle you posted here

That is what makes this sight right
Brain teasers that take up the night

Thank you man, this is a tough one
I doubt ill find the right answer to this one

Gotta admit though if i do
I might just stew a bit in my brew
: )


Sent from my iPhone using Home Brew

Bad rhyming on my part
Im stumped

Pappy? You out there?

Im feeling like s shmuck




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ajdelange said:

This is an old favorite. You are given two metal cylinders identical in size, shape and weight and told that one is made of magnesium and one of osmium. They are covered in a tough epoxy paint so you can't see the metals, can't poke with a knife to see which is softer and can't bash with a sledgehammer to see which breaks (well maybe you could but we'll tell you that that is not the answer). What very simple test(s) can you do (no instruments) to tell which is which?

Click to expand...

Since the densities are not the same,
the dimensions must be part of this game.
To be the same shape and size,
looking the same to my eyes,
the Osmium must be hollow,
and therefore if you follow,
put them in water,
I think we oughta.
Will the Osmium float,
just like a boat?

I was just posting and you beat me to boasting although I was gonna say, if I may.
A tap on the metals one will ting the other will tong but not for long.

A father and son get in a car accident.
Ambulances take them to different hospitals in this event. The boy goes into the operating room.
Its a shame that car was going zoom zoom.
And the surgeon says, I cannot operate on this boy, he's my son!
Aren't riddles fun?

I swore that though it make you curse
I would not fill my posts with verse
Indeed the one of Os is hollow
But thinking that it floats don't follow
For if the thought it does is true
So must the one of Mg too
That weights and sizes are the same
Is the first rule of this game
Think about it for a moment
The answer will shine like a comet

snowtires said:

I was just posting and you beat me to boasting although I was gonna say, if I may.
A tap on the metals one will ting the other will tong but not for long.

Click to expand...

But you can't tap on them because they are covered with a tough epoxy paint. Furthermore the rules prohibit an instrument as sophisticated as a hammer. I did think that someone might think of an accoustic test and so checked the velocity of sound in the two metals. Wikipedia says they are exactly the same. Too much of a coincidence I think.

But let's brush away these objections. Which one tings and which one tongs?

snowtires said:

a father and son get in a car accident.
Ambulances take them to different hospitals in this event. The boy goes into the operating room.
Its a shame that car was going zoom zoom.
And the surgeon says, i cannot operate on this boy, he's my son!
Aren't riddles fun?

Click to expand...

mcp!

ajdelange said:

I swore that though it make you curse
I would not fill my posts with verse
Indeed the one of Os is hollow
But thinking that it floats don't follow
For if the thought it does is true
So must the one of Mg too
That weights and sizes are the same
Is the first rule of this game
Think about it for a moment
The answer will shine like a comet

Click to expand...

You are correct I was to quick
now I look like a bit of a dick
But now I think I might have gotcha,
It must be due to the mass moment of inertia
For a hollow shape of the same mass will have more,
So I think if you put them on the floor,
Give them a spin and when they're done
the first to finish is magnesium

The man from the antipodes
The answer very nearly sees
Moments for spin are, I suppose
To one another pretty close
But more thinking along that line
Will well the answer here refine

Tip them. The osmium will fall over sooner because its center of mass will change more quickly as it is tipped (assuming the mass is concentrated around the outside... you didn't specify but I'm assuming the osmium cylinder is hollow and not some kind of cellular structure)

There are certainly unusual ways to configure the piece (one of which would make mattd's answer correct) but the obvious one, that it is a hollow cylinder with caps is indeed what I have in mind.

However it is configured the center of mass is at the center of mass. The center of mass will move towards the floor at 9.8 m/s/s in either case but you are sniffing around in the right area.

The center of mass with respect to the ground (i.e. center of gravity) may well be equal when the cylinders are standing on end. However, as the cylinders are tipped over, the center of gravity will move until it reaches a point where it is outside of the cylinder. At this angle, without support, the cylinder will tip over and fall (whereas when the center of gravity is inside the cylinder, it will fall back onto its end when released).

This angle is different depending on the mass distribution inside the cylinder, though my answer may have been backwards depending on the proportions of the cylinders (height vs diameter)

In any case I'm pretty sure what you're looking for is a kinetic test for determining the distribution of mass in the cylinder - that is how you figure out which is which with the knowledge that the osmium cylinder is hollow. There are any number of ways you could do this (I think the spinning idea is valid as well, since a hollow cylinder will have greater rotational inertia) but it seems like you are looking for a specific way.

chocotaco said:

The center of mass with respect to the ground (i.e. center of gravity) may well be equal when the cylinders are standing on end. However, as the cylinders are tipped over, the center of gravity will move until it reaches a point where it is outside of the cylinder. At this angle, without support, the cylinder will tip over and fall (whereas when the center of gravity is inside the cylinder, it will fall back onto its end when released).

Click to expand...

The center of mass (gravity) depends only on the distribution of mass within the cylinder. Assuming this cylinder to be a pipe with end caps the center of mass will be on the longitudinal axis half way up (assuming it is vertical) and will stay there. As you tip the cylinder it pivots about some point on the lip of the end cap where it touches the table or floor. Assume you are tipping it to the right. If the tip is slight the CG will be to the left of the pivot point so the gravitational force acting on the CG produces a restoring torque about the pivot point so that if you let go it will tip back to the upright position. If, however, you tip it far enough that the GG, still located in the center of the cylinder, moves to to right of the pivot point the gravitational torque is now to the right and if you let it go it will continue to tip.

chocotaco said:

This angle is different depending on the mass distribution inside the cylinder, though my answer may have been backwards depending on the proportions of the cylinders (height vs diameter)

Click to expand...

Assuming a symmetrical consruction (such as tube with caps) the GG is at the center of the cylinder (half way along its length). The critical tip is then atan(r/(h/2)).

chocotaco said:

In any case I'm pretty sure what you're looking for is a kinetic test for determining the distribution of mass in the cylinder - that is how you figure out which is which with the knowledge that the osmium cylinder is hollow. There are any number of ways you could do this (I think the spinning idea is valid as well, since a hollow cylinder will have greater rotational inertia) but it seems like you are looking for a specific way.

Click to expand...

Yes, that pretty much the case. I gave the answer when I wrote:

Think about it for a moment
The answer will shine like a comet

Comet is quite a stretch for a rhyme with moment which should have made it clear that the answer has something to do with moments. First: what I consider to be the obvious answer: As the cylinders are of equal mass and dimension but of material with highly different densities the cylinder wall of the Os one must be much thinner than the wall of the Mg one (which could even be solid). The mass in the Os tube is further from the axis of the cylinder so that the rotational moment of inertia of the Os one about the central axis is greater. If you try to roll the tubes the Mg one will roll much easier. If you tip the table a bit and put the two tubes at the high end at the low end the Mg one will accelerate faster than the other.

If you spin them then the moment about an axis perpendicular to the longitudinal axis is involved. As the distribution of mass along the axis is approximately the same for either these moments will be about the same. If, OTOH, you put most of the mass in the end caps then the Os one would have appreciably larger moment and you would be able to see the difference in rotational behaviour.

If by 'spin' he really meant 'roll' then that is indeed the right answer. Even if he didn't mean roll we have to give partial credit for that answer.


Related (sort of) practical application: Your SO boils half the eggs in a new carton for deviled eggs for a picnic. As she intends to make the deviled eggs later she puts them back in the same carton but then can't remember which are the cooked and which the uncooked ones. How can she tell them apart (without breaking/cracking them)?

ajdelange said:

How can she tell them apart (without breaking/cracking them)?

Click to expand...

Attempt to spin the egg on the counter or stove. If the egg is hard boiled the solid insides will spin along with the outer shell (obviously). This solid mass will spin for a long time. If the egg is raw the soft insides of the egg will try to stay in place as the outer shell revolves around them, robbing the impetus of the shell and slowing the spin very quickly. It all comes down to mass and Newton's laws. The raw egg is not a solid mass, it is in effect 3 masses: shell, white, yolk. Each of these masses will attempt to stay at rest. The force applied to spin the relatively low mass of the shell is quickly nullified by the higher mass of the white/yolk that are at rest. Some energy still gets imparted to the white/yolk and the egg will spin very slowly after equilibrium is obtained, but nowhere near as fast as the solid hard boiled egg. If we had wrists like a robot and could spin the raw egg around for many revolutions we would be able to accelerate the white/yolk to a point where the raw egg would spin just as well as the boiled egg, but we don't, and I am beginning to chase a rabbit here....

With Physics on my mind, this is all I can come up with. Probably not a riddle, but a puzzler maybe?

What will happen to the hole when you spin this coin?

ajdelange said:

Comet is quite a stretch for a rhyme with moment which should have made it clear that the answer has something to do with moments. First: what I consider to be the obvious answer: As the cylinders are of equal mass and dimension but of material with highly different densities the cylinder wall of the Os one must be much thinner than the wall of the Mg one (which could even be solid). The mass in the Os tube is further from the axis of the cylinder so that the rotational moment of inertia of the Os one about the central axis is greater. If you try to roll the tubes the Mg one will roll much easier. If you tip the table a bit and put the two tubes at the high end at the low end the Mg one will accelerate faster than the other.

If you spin them then the moment about an axis perpendicular to the longitudinal axis is involved. As the distribution of mass along the axis is approximately the same for either these moments will be about the same. If, OTOH, you put most of the mass in the end caps then the Os one would have appreciably larger moment and you would be able to see the difference in rotational behaviour.

If by 'spin' he really meant 'roll' then that is indeed the right answer.

Click to expand...

I did mean spin but I think boh cases are similar. Just one is accelerating the other deceleration. When spinning Both will have the same friction actig on them but the Os will spin longer since it will have more energy to start with due to the higher Iz. I should have been clear that they should be spun to the same speed at the start.

You guys are making it tough to rhyme
But what the hell I've got a dime
It has a hole in it
Where once some metal did live and sit
Now metal is gone and so is it's weight
So if my dime is spun I believe the hole
- might upwards gravitate
The solid part and heavier part of the coin
Finds its moment on the edge where a horizontal surface does join
And with that foolish answer I'm out the door
You guys are a tough crowd.. it makes me love this place more

Wait a riddled singed
The dad, the son, the surgeon thing
The doctor was the boys mother
Pretty sure she weren't no other

Aj answered that one too.
But to be honest I didn't get the initials
tween me and you.

Dan said:

You guys are making it tough to rhyme
But what the hell I've got a dime
It has a hole in it
Where once some metal did live and sit
Now metal is gone and so is it's weight
So if my dime is spun I believe the hole
- might upwards gravitate
The solid part and heavier part of the coin
Finds its moment on the edge where a horizontal surface does join
And with that foolish answer I'm out the door
You guys are a tough crowd.. it makes me love this place more

Click to expand...

Exactly so.
Way to go!

where is the thread where they talk about real men don't post riddles?

Setesh said:

Exactly so.
Way to go!

Click to expand...

Wow! Yay!
I truly didn't think my answer was right
It was just me posting into the night

Dan said:

Aj answered that one too.
But to be honest I didn't get the initials
tween me and you.

Click to expand...

MCP = Male Chauvanist Pig

The coin thing is another one solved by considering rotational moment of inertia. Things like to rotate about axes that have minimum inertia. The means getting as much mass as possible close to the axis. In the coin case that is done by moving the hole away from the axis which implies moving metal towards the axis. I'm not willing to sacrifice a 50 cent piece to see what happens but thats my guess i.e. that the hole would move up.

MCP.. I get that now
It's a bit deep but I like your speak how

I'm seriously wondering if my drill bit's tough enough
To give an experience answer too the coin stuff
I am also quite full of blarney
But.. man, I do love that AjDelange
- is in with this riddle thread party!

There is now 1,589 post in this thread and I'm foolishly going to ask the same first question, possibly different wording-- but the same question as the first post.

9 balls that each look exactly the same
One weighs different from the others... Heavier or lighter it does not matter
You have a balancing scale. Think of the pictures often used for The Hall of Justice scale

I'll give you a hint. You can put four and four on the scale . It's not going to get you the answer. There are to many vaiables with that idea.. Just know it only takes two measuring's to answer the question.


This riddle is decided in two moves. Every time .

Dan said:

There is now 1,589 post in this thread and I'm foolishly going to ask the same first question, possibly different wording-- but the same question as the first post.

9 balls that each look exactly the same
One weighs different from the others... Heavier or lighter it does not matter
You have a balancing scale. Think of the pictures often used for The Hall of Justice scale

I'll give you a hint. You can put four and four on the scale . It's not going to get you the answer. There are to many vaiables with that idea.. Just know it only takes two measuring's to answer the question.


This riddle is decided in two moves. Every time .

Click to expand...

It would be easy to do in 2 if you had told me if the ball was heavier or lighter - (assuming heavier) put three of the balls on each side of the scale, take two of the three balls on the side that drops and put the one on each side of the scale, the ball on the side that drops is the heavy one. If in the first measure the balance stays level use two of the three balls that were not measured. If it is level after the second measure the heavy ball is the one that was not on the scale. If assuming lighter it is the same but use the ball(s) on the side that goes up.
I can not for the life of me think of how to do it in 2 measures without knowing if the ball is lighter or heavier - that to me has to many variables to eliminate in 2 goes.

I think I can do it in maximum of four:
1)Split into 3 groups of 3 - weigh 2 of the groups, worst case is the scale is uneven and you have eliminated the last group of 3.
2)Now make 3 groups of 2 - weight 2 of the groups, worst case again is the scale is uneven and you have eliminated the last group of 2.
3)Now you have 4 balls - Weight 2 of them and:
4A) If they balance: Replace 1 ball - if they still balance the odd ball is the last of the 4, if they don't balance you have just place the odd ball on the scale
4B) If they don't balance: Replace 1 ball - if they still don't balance the odd ball is the ball on the other side of the scale, if they now do balance you have just removed the odd ball from the scale

Dan said:

There is now 1,589 post in this thread and I'm foolishly going to ask the same first question, possibly different wording-- but the same question as the first post.

9 balls that each look exactly the same
One weighs different from the others... Heavier or lighter it does not matter
You have a balancing scale. Think of the pictures often used for The Hall of Justice scale

I'll give you a hint. You can put four and four on the scale . It's not going to get you the answer. There are to many vaiables with that idea.. Just know it only takes two measuring's to answer the question.


This riddle is decided in two moves. Every time .

Click to expand...

Rereading you post it would seem you have not actually asked a question - you have given us some information about the balls (number and one weighs different) and the tools (a balance scale), but have not actually asked us to do anything.
So the question to you is do you want to know which? ball is different or if it is heavier or lighter? or both?
I can get it down to 3 if you consider both which group is odd, and how it behaves (heavy/light):
Weigh 2 groups of 3.
Replace one group with the last lot of 3. Now you should know which group has the odd ball and if it is heavier or lighter:

• if the first measure is balanced then the last group has the odd ball and if it goes up or down the ball is lighter or heavier respectively.
• If the scales are imbalanced initially then if you remove the balls that were lighter and the scale balances out the balls you just removed have the odd and it is lighter, if they don't balance out the set that remained on the scale has the odd and it is heavier.

Now if you only wanted to know if it was heavier or lighter you have your answer in 2 goes, but to find the actual ball requires one more - weight 2 of the 3 balls in the odd set :

• If they are equal the left over ball is the odd one
• if they are unbalanced then you know which is the odd depending on if it was determined to be heavier or lighter above

I'd so the two groups of four. If balanced the one left out is the heavier/ lighter ball.
If one side is heavier/lighter split that group on the scales. And again repeat with the heavier/lighter group. Is the only way I see to work it out but that still takes 3 weigh ins I don't see it done in two.


Sent from somewhere to someone

If we knew if the odd ball out was heavier or lighter you would weigh two groups of three balls. Choose the group that is heavier or lighter depending on what we are looking for or in the case that they balance, choose the remaining group of unweighed balls.

From the chosen group weigh one ball on either side. The (heavier or lighter, once again dependent on what we are looking for) one is the one you want, unless it balances, then it is the remaining unweighed ball.

I'm assuming dan means it doesn't matter for the sake of the problem whether the rogue ball is heavier than the others or lighter than the others, but that whichever it is, you as the weigher would know whether you were looking for a heavier ball or a lighter ball.

If the premise really is that you only knew one was different (and not whether it was lighter or heavier than the others), pretty sure you can't guaranteedly solve in two moves with anything more than three balls.


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Holy crap. I am an idiot. I ruined this riddle with bad information. The odd ball is heavier.

Matt, thank you for bringing my error to light. I totally made a fool out of myself.

What three letter English word has an odd start, an even finish and an infinitely long middle?
The end word being middle makes it easy to rhyme this riddle

Knowing whether it's heavier or lighter makes it trivial. It is solvable in two moves if you are lucky and wind up with the odd ball in the group of three that is not weighed in the first move and then get lucky again when you pick two from the remaining three. IOW if the pans balance on the first and second weighs then the odd ball is the one that never got weighed. Odd are 1 in 9 that things will turn out this way.

So now the question becomes "Can you handle the extra degree of freedom (is the odd ball lighter or heavier) with one more weighing?" Answer is 'Yes'.

LabRatBrewer said:

What three letter English word has an odd start, an even finish and an infinitely long middle?

The end word being middle makes it easy to rhyme this riddle

Click to expand...

Wow! You raised the notch!
Riddles like this always leave me car Blanche. It rhymed : )

I'm sure somebody will give the right answa
Even Dasher is vexed and only prances.

Dan said:

Wow! You raised the notch!
Riddles like this always leave me car Blanche. It rhymed : )

I'm sure somebody will give the right answa
Even Dasher is vexed and only prances.

Click to expand...

Your answer hides how clever it is
Riddle Answerer, you are a wiz

LabRatBrewer said:

Your answer hides how clever it is
Riddle Answerer, you are a wiz

Click to expand...

yes, it is a very vexing rhyme