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.


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

  2. 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.

    1. 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

  3. 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!

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

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

  6. 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.

  7. 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.)

    1. 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.”

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

    Still, nice start to a Friday 🙂

  9. 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.

  10. 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.

  11. 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!

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

    1. 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.

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

  14. 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.

  15. 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.

  16. 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?

  17. 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?

    1. 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.

    2. 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

  18. 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?

  19. 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!!!!

  20. 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.

  21. 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.

  22. 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.

  23. 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 !

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