It’s the Friday Puzzle!


Galileo pointed out that, ignoring air resistance, all bodies fall with the same acceleration.  But what about objects sinking in water of different temperatures?  Here is the puzzle:  Suppose you drop a brick in each of two identical tanks.  One of the tanks has water at 40 degrees F and the other has it at 30 degrees F.  Which brick would sink faster?

As ever, please do NOT post your answers, but do say if you think you have solved the puzzle and how long it took.  Solution on Monday.

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.

108 comments on “It’s the Friday Puzzle!

  1. Kristian says:

    Interesting science puzzle, I like it.
    Don’t have an answer just yet, I’ll think about it🙂

  2. Michael Sternberg says:

    Solved it in about half a minute, contemplating possible slip-ups from the fluid dynamics treatment, but I’m fairly solid on my answer.

  3. Kristian says:

    Ah got it now, a couple of minutes during breakfast, I like the solution🙂

  4. M says:


    Not used to Fahrenheit, so had to check.

    Know the answer now.

  5. Steve Jones says:

    I’ve got an answer, but it involves neglecting some (I hope) minor effects like the change in water viscosity with temperature.

  6. Brian says:

    I needed a second read, but worked it out in about 15 seconds. More of this type of puzzle please!!

  7. Dan says:

    Got it! In about 20 seconds!

  8. francpaul says:

    that’s just too easy. fun.

  9. Tom Pedersen says:

    Not being used to Fahrenheit, I had to convert them to Celsius first.

    After that, it took me about 2 seconds.

    I assumed pure H₂O at one standard atmospheric pressure for my… calculations.

    • Anders says:

      You shouldn’t have assumed that. If the water is pure, it has very different properties compared with ordinary water in nature.

      My answer is that the information given is insufficient, but I can guess what the sought after answer is

  10. Simon Taylor says:

    Not very hard, this one.

  11. About 5 minutes. Should have been 5 seconds!

  12. Dave says:

    Got it before I finished reading the question.

  13. About 5 minutes. Should have been 5 seconds but it is cold and wet this morning…

  14. safc4ever says:

    I git this one in about the same time as the faster brick takes to fall to the bottom of a domestic water tank.

    It didn’t take long for the penny (or brick) to drop!

  15. S says:

    Don’t really need to take fluid viscosity into account. Don’t need better than a high school understanding of physics/chemistry.

  16. Paul Durrant says:

    Ten seconds to get the answer, once I read the question fully.

  17. Suzanne says:

    Must read the question more carefully in future
    Must read the question more carefully in future
    Must read the question more carefully in future…

  18. John says:

    Ill be honest I don’t normaly get these but I think I have this week. Excited for answers on Monday now

  19. Paul Creevey says:

    Yep. Tool a second for the penny to “drop” (below)
    Good one & obvious when realised!

  20. RobHes says:

    1 second, for once. Maybe showing my age

  21. Once I remembered certain significant temperatures in fahrenheit rather than centigrade, it took very little time at all the calculate the descent rates. A good science problem. One to use at school.

  22. Wouldn’t have got it, had I not read the comments. Please folks, resist the temptation to give away the answer! (And yes, I know I should have resisted the temptation to read the comments when I got stuck.)

    • M says:

      Saying that it is easy is giving it away in this case i guess… So, hard not to give it away, since “do say if you think you have solved the puzzle and how long it took.”

  23. Took a brief detour down a mental blind alley, re-read the question and laughed.

  24. JP says:

    Agreeing with Catherine…Those comments basically tell you the answer, rather than hinting at it..

    Still, nice start to a Friday🙂

  25. Neal Ledwith says:

    I loved this puzzle, brilliant question. Only took about a minute for the solution but much complicated theorising till the answer suddenly arrived.

    Got the grey matter and laughing gear working for the day, thank you.

  26. JimC says:

    Easy Peasy – which is nice, as I’ve not been doing well lately.

  27. Yeah knew this one already, though as 30C vs 30F

  28. About 5 minutess. Should have been 5 seconds – but a bit cold and wet this morning.

  29. Anonymous says:

    Got it in the time it took me to convert Fahrenheit in Celsius (thanks to Google).

  30. AMWhy says:

    I got an idea for a solution in about a second. Hope I’m right but need todo some research to find out. Google time!

  31. Andy Holding says:

    Yeah good one. Took me a couple of seconds. I like these kinds of puzzles. Will follow on twitter and look forward to more of the same.

  32. Paul Clarke says:

    Tried the tested formula…keep it simple and the answer is usually in the question…took about 40 secs…good one!!

  33. Al (@Al__S) says:

    Got it as soon as I noticed the temperature units…

  34. Rhys Morgan says:

    Given that I’ve just been doing about the chemical properties of water in my biology AS course (biological molecules topic!), I just converted to °C and applied my knowledge to the results!

  35. Timdifano says:

    Nice one! I had a solid answer too, and this avoids having to consider factors like turbulence…

  36. Chris says:

    I hope you don’t set questions like this for students. Far too ambiguous and non-specific. There could be several answers.

    • I think this is exactly the kind of question to set to students. It helps them learn how to think, consider different possibilities, and read questions thoroughly.

    • Anders says:

      Learn to think and arrive at the wrong conclusions, it looks like. It seems everyone here is convinced that just because the temperature is -1° Celsius, the correct answer is that one tank is ice.

      The problem is that it won’t be ice.

  37. It wouldn’t work so well with a US audience, would it?

  38. A European brick, or an African one?

  39. Anonymous says:

    Well, I ran it up the flagpole and got the answer in 0.0830149875491625349172635496391654915465639196753976549176549726549710 seconds

  40. Got it straight away, although I had to assume the air pressure was 101,325 Pa.

  41. riffy says:

    You bugger! You had me thinking about it for a few minutes before I got it!

  42. astro says:

    the answer very much depends on either the purity/salinity of the water, or the temperature of the brick.

  43. drgeraint says:

    Got that the moment I saw the temperatures were given in Fahrenheit. Then checked the conversion to Celcius to be sure.

  44. Tempting to make a joke out of how I got it–but “I see” you want no spoilers here.

  45. ChasTiv says:

    Sea water would have made the answer much harder to find.
    Wot with viscosity, density of both brick and water to contend with, not to mention numbness in fingers as you fished it out for another test.

  46. Anonymous says:

    Can’t usually say this about Richard’s excellent puzzles, which usually take me between a minute or two and not at all, but I got this one instantly. Feeling very pleased with myself.

  47. mittfh says:

    Easy peasy lemon squeezy🙂

    2 seconds, if that!

    Is it saying too much to say that under standard atmospheric pressure, pure water behaves quite differently at the two temperatures?

  48. Lazy T says:

    A red hot brick or an ice cold one?

  49. Tonybargin says:

    Ooh I finally got one! Woohoo

  50. TS says:

    Very hard puzzle, the answer is slippery, yet I kept my cool and figured it out in zero seconds.

  51. Ed says:

    I knew what was coming before I finished reading the question. And yes, it’s easier for those this side of the pond.

  52. z says:

    You mean a construction brick, right, not a brick of some other substance?

  53. Anonymosity says:

    Solved before I finished reading the question.

  54. Rayne says:

    Got it in about 2 seconds after I realized he said F not C

  55. Roy says:

    I got AN answer almost instantly.

    Always suspicious of the quick response but I can’t find a problem with it

  56. Physicalist says:

    I believe I got it. Ten seconds.

  57. deepfield says:

    Trivial puzzle. Longer to read than to solve.

  58. Anders says:

    Based on the density of water at different temperatures, it took about the time it took to look up the densities, a few minutes. At first I thought the information given was insufficient, but then I realized that since the tanks are only compared to each other, it is enough.

    To those who solved it quickly, have you seen videos like this?

    • wagdog says:

      So what really matters in the roughness of the brick providing sufficient nucleation sites.

    • Michael Sternberg says:

      I’ve actually seen this very video before and noted the somewhat rag-tag rag beneath. It is a nice tangent to explore.

      But the puzzle is about which version is faster. Are you positing that the order changes due to the effect demonstrated? I believe it does not, regardless of the manner the effect manifests or if we just assume the canonical outcome.

    • Anders says:

      No, I am positing that the question supposes that water at -1° Celsius is ice – a trick question. The video, showing liquid pure water at minus 20 something Celsius shows that it’s not the case for water that cold, it certainly won’t be for water at a mere -1, so it the answer is a prosaic one, depending on the density of the (liquid) water

  59. Kristin says:

    Ah, yes. Got it as soon as I read the question, for once. *celebrates with coffee*

  60. Hal says:

    This was not as much a traditional puzzle as it was a test of one’s knowledge of physics. Very easy for me.

  61. Mike Clayton says:

    Definitely one with an age or cultural bias.

  62. rmb says:

    I thought of one answer while reading the puzzle. Then while reading the comments, I realized what I had overlooked and got the correct (or expected) answer. Then thinking further while reading more comments, I wonder if the data is correct. Say, would the behavior be same at sea level and at much higher altitude where the atmospheric pressure is significantly different?

  63. Camel Ali says:

    Quickly got an answer. Then the penny dropped. My answer is still right, but for different reasons.

    Nice to have a puzzle that actually does have an answer this week!!!!

  64. Jerry says:

    I got this one immediately. Is there a trick that I’m missing (other than the obvious one)? Tune in on Monday for the next exciting installment.

  65. Super Cheese says:

    It’s an interesteting one but there’s clearly two answers that can be given as the question doesn’t tell you anything about other, significant, factors – pressure being one of the most obvious here.

    But a nice wee one to get people thinking.

  66. Ruddigore says:

    It took me three minutes… it should have taken me three seconds!

  67. @davidbrandwood says:

    Come on not that old chestnut.

  68. mklprc says:

    It took about three seconds. It would have been a more interesting question if it had asked for water at 35F and 75F.

  69. Mr. D says:

    Ah I see! Got it in about 5 seconds. Good puzzle.

  70. SteveG says:

    Seriously? Is this really a “puzzel”?
    It took me as long as it took to read the question…

  71. Sari1967 says:

    Bit obvious this one!

  72. Kriss says:

    Got it. 3 seconds. Personal best!

  73. Edgar says:

    I’m not used to Farenheit, so I had to check. Not really a puzzle, but a science test.

  74. One Eyed Jack says:

    This is the kind of question I would expect to see on “Are You Smarter than a 5th Grader?”

    Let’s get something more solid next week.

  75. JK Grence says:

    Think, think, think, theorize, OH GOD OF COURSE.

  76. spiderabc1 says:

    Think I have it. Popped into my head just after reading.

  77. Got it while reading. However, I will never understand the use of the Fahrenheit scale.

  78. Mark says:

    Unusually easy for one of your Friday puzzles!

  79. Seems very easy… 10 seconds no more! Is there a trick though? And yes, please use Celsius and the metric system in general.

  80. The other Matt says:

    Damn, got it now suddenly… ! But first i spent a couple of toilet sessions to think about details (There are also “Bricks” sinking, haha). But im sure i got the “other solution” too !

  81. Fabrice B says:

    About 1 mn. I am not used to Farenheit.

  82. Tomble says:

    is it a Lego brick???

  83. Anonymous says:

    Got it, say about 30 seconds, once i rembered about the anamolies of water🙂

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