4 Revolutionary Riddles Resolved!
100 Comments


This video contains the answers to
my four revolutionary riddles, so if you haven’t seen the riddles yet, you should probably watch them before you watch the answers. It’s OK; I’ll wait.
Just click this card up here. [Ticking clock sound] Now, when I filmed the riddles,
I also filmed the solutions at the same time, but that was before I received your 15,000 comments and dozens of video responses, so I’m re-shooting parts of this solutions video to incorporate the results I saw in the comments and to use some of your video responses to help explain the solutions. Let’s get to it! OK, by looking at the comments, for #1,
over 15% of you said “a cylinder containing sand”, Now, this contains some powder. [Chuckles]
See how it rolls… It, uhh, it doesn’t seem to roll… …very far before it stops,
and then it won’t roll again because I think the sand just kind of levels off in there. Maybe you were thinking a bigger kind of sand, like these small gravely stones. Let’s try that. That actually rolls pretty well. Nearly 25% of you said,
“a cylinder half-full of water,” so let’s try that. This rolls very well, so it’s not water… and nearly 45% of you said,
“a cylinder half-full of a viscous liquid.” Here, I have a half-full container of honey, so let’s see how it rolls… That’s not bad;
it’s rolling and it’s stopping, and it’s rolling some more… This is a pretty good guess, and I think the behavior is not exactly the same as the mystery cylinder, but it definitely is similar, and that’s no coincidence. The mystery cylinder actually contains honey… and ping pong balls. There are two ping pong balls submerged in this honey. So if I place that on the ramp, the center of gravity is not above the point of contact with the ramp, and so it rolls forward, but now, because those ping pong balls are in the front, they change the center of gravity, and so it’s exactly over the point of contact, and so it stops briefly, but then, as the viscosity of the honey allows those
ping pong balls to move up, the center of gravity shifts forwards again,
allowing this little container to roll. So that is the trick of the mystery cylinder. Pretty easy if you want to try it out at home. Now, I challenged you to run two laps of this track,
where the first lap, you could go as slowly as you like, but the second lap, you had to go much faster, such that your total average speed was twice
the speed of your first lap. Now, when I was first asked this question
by Simon Pampena, it took me a long time and scribbling on paper,
and just something didn’t seem to work out, and that’s because… you can’t actually do this. It’s impossible. I mean, you might think I could run 3V₁ for my 2nd lap, and that would mean my total average speed is 2V₁. The problem is you can’t just add the two speeds together and divide by two, because you spent much more time in your first lap, so that speed is weighted more heavily into the average, so you’d have to run, well, impossibly fast. Let me explain. The velocity of the first lap was the distance around the track divided by t₁, the time it took you. Now, if you want your total average speed to be 2V₁, well then it needs to be 2d ÷ t₁. You need to run twice the distance in the same amount of time it took you to run the first lap, but you’ve already run that first lap, and so you have
no time remaining to run the second lap! Even if you went the speed of light, you would not be able to increase your total average speed up to twice the velocity of your first lap. It is just mathematically impossible! So this may seem like a bit of a trick question, but the point to me is how doable it sounds,
how it seems like something you should be able to do, but you can’t.
It’s actually impossible. Riddle #4, the question about the train, was actually answered pretty well, with most people mentioning something to do with the wheels, but of course, that makes sense, in a series of riddles which are about rotation, rotational motion. Some people though did point out that maybe it was the steam that was going backwards, or maybe air molecules in the train,
and that is actually a pretty clever point, however I wouldn’t really consider the air in the train part of the train, so indeed, the part of the train that is moving backwards is the flange part of the wheel,
which is below the rail. That is the part of the train which is moving backwards. To understand why,
you just think about a spinning wheel. The top of the wheel is moving forwards at speed 2V, and the bottom of the wheel is not moving forward at all; it is stationary with respect to the track, and that is what we call “rolling without slipping,”
and that’s how most wheels work. At least, that’s how they’re designed to work. Now, in the case of trains, they have to have flanges so the train doesn’t fall off its rails, but of course, when these pieces come around during the rotation of the wheel, they actually extend beyond the rail, and therefore,
they are going backwards with respect to the ground, so the part of the train that’s moving backwards is always changing, but it’s always that part of the flange, that part that extends beyond the wheel that is below the level of the track. So what happens when you pull
the bottom pedal of a bike backwards? Well, about 45% of you thought that
the bike would move backwards, about a quarter said it would move forwards,
and a quarter said the bike wouldn’t move at all, and 5% said it depends on something… So let’s give it a shot and see what happens. I’m going to pull backwards on the bike pedal
in 3, 2, 1… Woah! The bike did indeed move back, and for virtually all bikes, this is what you will find, but the explanation is not just as simple as
“well, the net force on the bike is back, so therefore, it has to accelerate backwards,” and to prove that that logic doesn’t work,
well, just have a look at this video by George Hart. [George] Watch this.
Again, I pull the same pedal backward, but now… the bike moves forward! [Derek] I’ll put a link to the full video here
and a link to his website in the description. So the reason the bike moves backwards is because of the way these gears are set up, the diameter of the tire, and also, the distance from this crank to the pedal itself. Because as a bike moves forwards, the pedal, even when you’re pushing back on it, never actually moves backwards with respect to the ground; it’s always moving forwards. [George] So if you drag a string
behind the pedal of a bike moving forward, the string is always moving forward. Now just play that movie backward in your mind, and it may be clear how pulling the string backward could make the bike move backward; They move forward together,
so they move backward together. [Derek]
Another way to think about this is to consider the path traced out by the pedal
as the bike moves forward. This is called the trochoid. For all ordinary bikes,
the pedals are moving much slower than the tires, so the pedal is always going forward
with respect to the ground, but George modified his bike so the ratio of the pedal to the wheel radius was greater than the ratio of the front sprocket to the back sprocket, and this ultra-low gear changes the trochoid
so the pedal DOES actually go backwards with respect to the ground as the bike goes forwards, and that’s why he could pull back on the pedal and make the bike go forwards. This is the same reason why if you were to pull backwards on the flange of a train wheel, you could actually get the train to move forwards,
if you pulled backwards with enough force. For all normal bicycles, pulling back like this on the bottom pedal will cause the bicycle to move backwards, but, depending on the gear ratio,
you can get the bike to move forwards, so it was those people who said it depends on the ratio of these gears and the size of this crank to the radius of the back wheel that were actually the most correct.

100 thoughts on “4 Revolutionary Riddles Resolved!

  1. Theoretically, if you didn’t run a lap at all, then T1, T2, and D would all be 0, and the average of this would be 0, which is technically 2 times 2D/T1

  2. My dad started his career as a math teacher in the early '60s. He posed problem 2 to his students (although it was a car, and he dictated V1 as 30mph and Vavg as 60). I got it right, and I've been asking it of friends and colleagues/subordinates ever since, and only about 5 people have gotten it. The stubbornest wrong was actually a statistician from the CDC. He kept insisting that the average of 30 and 90 is 60. I agreed, but I couldn't convince him that's not what's being asked. We can only hope he's not in charge of the smallpox stockpile.

    By the way, you can tell whether you're asking an engineer or a physicist. The physicist says you have to go infinitely fast, and the engineer says you can't do it.

  3. mystery #2 what if you point a laser in a blackhole? trick for distance is you get closer to start with and into the blackhole and youll get above v3

  4. ok the first one wasn't really a riddle so much as a blind guess. Who on earth has ever seen a jar of honey containing some sort of globe rolling down hill?

  5. I hate to be that guy but I have to call BS on the train riddle. You said the ground, not the track. The track is above the ground which means that the flange is, at most, stationary in relation to the ground. If the flange in the ground then I would give it to you (even though you could get more technical by questioning at which you would consider the "ground" to end)

  6. 2 years later
    If you draw a point in any part of a train wheel, it won't move backwards instead I will decelerate but always forward.
    Do I miss something?

  7. The runners question IS a trick question imho. Its just a mathematical problem, not a physical one.
    You can run a lap with lets say 2km/h and then run the second lap with 6km/h, which give us 3*V1.
    The question was to run that lap so that Vavg from both laps is 2V1 and thats it.
    If you have 100m lap, you would have been running for 3 min and for 1 min.

  8. The truth is when you guess at what you know and many do even balls to defend it as a know when wrong and guess usually is you become a lying cocksucker defending lie as truth . Many do ate and clueless to it is just Injust and dishonest drives me nuts and many many do exactly this . Refuse to check or hear anyone just defend there lie or someone else's lie refusing to look . Exspecially all your anti American news and leader . America cornerstone and it dies without it is unbaised unopionated facted checked news all Americans supposed to know this and know how to check it for facts. Do you still lying cocksucker or anti American

  9. Referring to the track problem : When you ask for average speed why does time become a part of it?? It was not part of the question. So first round 1 km/h second round 3 km/h and you get an average of 2 km/h

  10. Hmmm with the bike there is 1 correct answer.
    It falls down.
    It won't go forwards or backwards because of the string, it will fall to the ground no matter which way you pull it.

  11. At my last internship I gave the oval question to 3 people I was working with. Two of them accepted the answer (although one continued to argue that 'infinite speed' means that there would still be an increase in time because infinity is "just a large number" *smh*) and one struggled with it the whole day. She came back to me the next telling me that the "question is wrong… that everyone had misinterpreted the question… needs more information provided… ect and that 3V1 is the right answer." I did everything to show why this wasn't the case, even go so far to give her proof with actual velocities, times and distances. Still no progress. She was a school teacher in the past.
    Moral of my experience: Kids when show a school teacher something outside their field of teaching and they tell you that you're wrong don't let that get you down. You very well could be right… especially if it is a relatively complex subject.
    Even as an someone studying masters and completed honours in engineering I found this so demoralising and soul sapping that someone could be so stubborn and irrational that they wouldn't accept any proof. Probably sheds some light on why its so hard to for engineers and other scientists to address serious world problems too.

  12. 3:08 What are you talking about???? Like really????? Let's say the track is 300 m if you took 3,600 seconds to do the first lap you'd have a velocity of 1/12th m/s. If you did the second 300 m lap in 1,200 seconds you'd have a velocity of 1/4th m/s. If you average the two you get 1/6th m/s, aka 2/12th, aka 2 * v1.

  13. I would like to argue that honey with either ping pong balls or air "bubbles" could be viewed as pretty much identical. Apart from the shape of those air bubbles and some added friction from the balls.

  14. the lap running thing you asked the question in a very wrong way, and here running around lap you fking state formulas …. bro your videos are random with your own answers …

  15. take the fking gear out of the bike of you gearless bike and then do the string, this guy sis think if ppl didint know this basic logic physics then

  16. First lap 10 km/h, second lap 30 km/h. Average is 20 km/h, thus twice that of the first lap. Problem solved.

  17. You should run in a constantly expanding track, so the second half of the track will be longer than the first, then will be fhisically possible to increase your average speed to the double if you can run the second half fast enough.

    Other way is, to run the first half at v1, then run the second half at 3v1, then before stepping to the finish line, turn arround and keep runing at 3v1 till the middle of the track, at that point, turn back again and run to the finish line at 3v1.

    You need more space to run at the second half than the first.

  18. if anyone knows a visual physics lab program for windows to visualize physical phenomeno please let me know

  19. question abut the velocity is wrong.
    If u r 10km/h for 1h and u go 30km/h in another hour than u went 40km in 2h so average is 20km/h and it is twice as fast as u went in the first hour.
    u make your question wrong or u cant understand what u r saying

  20. Please don't give wrong explanations here. This is one of the most illogical reasoning I can see at play. Let us assume an object covers a distance of 100 m in 10 sec. So, the speed is 10 m/s. Now let us say that the object travels 300 m in the next 10 sec. The average speed of the object is total distance travelled / total time = 400 / 20 = 20 m/s, exactly twice to what it was.
    Furthermore, there is a difference between speed and velocity. Please use the terms properly.

  21. Wait…for the track question:
    If I somehow found an infinite amount of energy and ran with c, time would stop for me, wouldn't it? Thus t2=0 (admittedly, only for me but he didn't specify a POV)
    Soooo….light could make it…from the lights POV
    And even if I were to travel slower than c, as long as my lap is less than a Planck-time it would still be 0!
    Take that simple puzzle XD

  22. So if I was to truly answer the question I would in one of 2 ways: the first answer is I would run the first lap in 1 min and the second lap (in order to meet the criteria) I would run it in my mind instead of real life. Please think about how in real life, if you have two 1 lap races you're competing in and the judges are calculating average speed. You can choose to get the right result by simply sitting through the second race on the bench and running it in your mind only which would give you the zero you need. The second and more favorable answer is: I ran the first lap in 857 strides per minute which took 1 minute. I reduced the length of my stride by 3 and ran for another lap at 2571 strides per minute which took 1 minute. My average total flesh SPEED for both laps is 1714 strides per minute…my average speed was twice as fast as the first lap. My legs can move faster but that doesn't mean my body has to! #semantics because everything is a matter of perspective.

  23. That would be like saying the wheel is moving backwards to the ground the flange he is part of the wheel cast all in one piece

  24. I mean… half your viewers said honey and air. Then you try to say that was "close" and the answer was honey and air-filled pingpong balls. Then you said velocity of 2 laps, but never said you had to actually calculate in the time the lap took. Someone can run 5mph without actually running a mile. :rolleyes:

  25. Did anyone point out that the flanges don't normally keep the train on the track and… how can the train go around a curve since the axles and wheels are a solid, one piece?

  26. We could run at the speed of light in the second lap as when an object's velocity is equal(or almost equal) to speed of light the time stops.
    Special theory of relativity!!
    But unfortunately reaching the speed of light is impossible.

  27. Well if it took you say 200 seconds to run a lap with an average speed of 5mph, it seems completely logical to me that if you cut your time in half, it would double your average speed.

  28. Technically velocity for both the laps was zero because of zero displacement..you must be referring about speed which is possible to get 2 times avegare speed by running at 3 times speed of First lap.

  29. Though I am not sure that the Relativity of Einstein is true or not. But according to it, if any object gains The speed of light, the the space becomes shrunk into 0 relative to itself. So , if we take the velocity of light, we could do that.

    This is just according to Einstein's Special Relativity.

    I am not sure about the truth.

  30. #3: 2 laps at the track. Run the first lap, 400 meters, in 8808 seconds. 400/8808 = 0.0454. Run the second lap in 80 seconds. 800/8888 = 0.0900. 80-second laps are challenging, but maybe not as challenging as 8808-second laps are.

  31. I am late do find this video, but have resolved the track problem with the following solution, run the first lap in the inner track in a time of 525,600 sec (V1 = 400M/525,600 sec = 0.000761 M/sec),run the second lap in the outer track in a time of 34,400 sec (453M/34,400 sec = 0.01162 M/sec). Avg speed is 853M/560,000sec = 0.001523 M/sec which is more than 2x V1. Thoughts?

  32. So why the train doesn't fall off its rails? Flanges are not the answer – it is only an emergency system. There is something more tricky.

  33. Veritassium committed a *BASIC LOGIC ERROR* He confused time for a 2nd (independent) lap with some
    sort of "total" time, which is not part of the original question. Nobody is running "twice the distance" 2 d.
    The error is in trying to interpret 2 V1 = 2 d / t1 (an intermediate step) without following through on the rest of the algebra. He even had to edit the video to correct conflating "average time" and "average speed" at 3:40. From there, it's just a descent into the rabbit hole at the speed of light. The ability to pick apart the essential elements of a math problem is rooted in thorough mastery of spoken language.
    Solve the easy way: V1 = d1 / t1. Set V2 = d2 / t2 = d1 / t2 as d2 = d1.
    Define Vavg = (V1 + V2) / 2.
    If Vavg = 2 V1 = (V1 + V2) / 2, then have 4 V1 = V1 + V2. So V2 = 4 V1 – V1 = 3 V1.

    Or this way: Define V1 = d1 / t1 and also as d2 = d1 then V2 = d2 / t2 = d1 / t2.
    In order for Vavg to be 2 V1, then Vavg = (d1 / t1 + d2 / t2) / 2 = 2 d1 / t1 = (d1 / t1 + d1 / t2) / 2
    Divide out d1 and multiply both sides by 2 to get 4 / t1 = 1 / t1 + 1/ t2 or 3 / t1 = 1 / t2 giving t2 = t1 / 3 or v2 = 3 V1.

  34. Seems to me that they weren't the "most" correct but that they WERE correct.

    Type of person who doesn't like to admit another does well. Only mostly (or kinda) well.

  35. Bruh the one with the lap is actually stupid of course the speed will affect the time you spend running on that velocity cause the distance stays the same. When you presented the problem you made it sound that it was about the average of the average lap speeds not the average speed of the whole time running.

  36. When the distance is same, (sp. case)the formula for average velocity will be 2xy/(x+y),where velocities are X and Y units. So let's consider my 1st round velocity to be V. And i want my Avg
    Velocity to be 2V. Then considering my 2nd round velocity to be 'W', i will get :-
    2V = 2VW/(V+W)
    => V=0, means this is impossible… ??

  37. I commented on the original video when it came out 2+ years ago and I'm just seeing this now. I was right that it depends which gear the bike is in. But I was wrong about the gear ratio, I thought 1:1 was going to be the dividing line but it's much lower than that.

  38. Actually, if you are a man running down hill in a lose fitting pair of shorts, you have to stop every once in a while to adjust your balls, simple really when you think about it 🙂

  39. (Answer to the 2nd question) According to the General Relativity, if you move at the speed of light, time does not pass which means you can finish your second lap in no time and in this case, your average velocity would be twice the V1 :))

  40. the bike tests lol. the 1st he pulls the bike backwards with his hand on the sit you can see it because if he pulls the pedals the backwheel can only rotate forward and it rotates backwards, the 2nd he pulls so hard that the wheel doesn't grip so of course the bike its coming backwards and the 3rd he doesn't even pull you can see the pedals goind backwards. the only test done correctly its by the other guy and the bike moves forward.

  41. I thought Newton Fluid and V1 (1mph) and V2 (3mph) which would make the average of V1 and V2 double the speed of V1……but he explained it is theoretically impossible…..so i guess i'm wrong…..don't comprehend how he explained it but sounds like i'm wrong.

  42. wrong about the velocity question as you only talked about the velocity variable with out including the time in the question. so while I get that in this video you explained it fully, and this question is impossible. the previous question being only velocity is simple and easy, 1mph to then 3mph which would avg to 2 and be twice that of your first laps velocity…

  43. Actually the flange on train wheels does not keep it on the track. What keeps the train on the track is the fact that the wheels are not flat but conical so the train can go around corners with one wheel moving faster than the other and have a solid axle. The flanges help the wheels over points. This is a very common mistake, I used to be an NDT inspector of wheels.

  44. in the 2 lap question instead of doubling the distance why can't we half the time for the same distance,, by running the same distance in half the time should yield us the same result, pl correct if im wrong !!!

  45. Question 5 : what happens if you tie string to a cats tail ?
    & pull backwards? ??

    Answers in comments pls……
    & I shall pick the right answer (& please NO practical demonstrations!)
    ?. ?

  46. I really don’t understand the solution (or not?) to the 2nd question.

    Yes, 2*V1 = 2d1 / t1
    But, you can also say that 2*V1 = d1 / 0.5t1

    So, instead of doubling your distance, you can just run the same distance in half the time.

    That (theoretically!) means for example: I run 100 m in 5 seconds, that means my velocity is
    V1= 100m / 5s = 20m/s

    2nd lap, I run 100 m in 5/3 seconds, that means
    V2 = 100m / (5/3)s = 60m/s

    (20m/s + 60 m/s) / 2 = 40m/s

    Please show me where I’m wrong, I really don’t understand it…

  47. for Q1 I thought it was a magnet wire + magnet; Q2 is quite easy if you do a little math, it's mathematically possible as long as v2 is infinitely large, which means it is physically impossible (if light speed is indeed the fastest a particle can travel); train question is easy. bike question is also easy for anyone ever rode a mountain bike climbing uphill with gears all the way down

  48. The air is part of the train. Once movement occurs it's part of a 'system.' Similary, would a bird-delivery truck weigh less if all the birds were in flight throughout the duration of tge trip – no because the air inside forms part of the system.

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