At the Palace of discovery in Paris, they have this huge turntable where you can sit, and perform experiments. Like in the middle of the turntable you can put some water and then add liquid nitrogen, and this creates a kind of fog these tiny water droplets that spread out from the middle, but because the actual linear speed of this turntable is increasing the further out you go well, the vapor falls further and further behind creating a swirl, that looks a little bit like a Hurricane. And that analogy is not by accident, the Earth is after all one giant turntable, and the weather patterns we get are a result of that. When you study Physics in rotating frames of reference, or even just rotating bodies. You find some pretty mysterious and counterintuitive phenomena, which is why I'm dedicating this video to four revolutionary riddles. Number one here I have a ramp, or an inclined plane and just to show you i'm not doing anything funny with it, if I just let a roll of tape go it will accelerate down the ramp. But! Here I have a mystery cylinder, and I'm going to place that on the ramp, and we will see that it rolls, and then it stops, and then rolls, and then stops. So my question to you is. What is In the mystery cylinder? What is this object? Number two, take a bicycle and have someone support it, so it doesn't fall over or lean it up against something, and then take a bit of string and tie it to the bottom pedal. Now what I'm going to do is pull backwards horizontally on this string, and I want you to predict what the bike will do. Will it go backwards? Forwards? Or will it just stay there and not move. My intuition would tell me that it would go forward? I think it might not move if you're going to be pulling backwards from a stationary point on the pedal, you're going to be pulling it backwards just as the pedals are tying to spin the wheels forward, so maybe it might not move. Things gonna go forward, I'll go forward. Sure? Yeah, I'm sure. all right three, two, one. Number three. I want you to think about running two laps of this track now the first lap you can go as slowly as you like, let's call that speed V1. But on your second lap, I want you to run faster. Such that the total average speed for both laps combined is twice V1, twice the speed of your first lap so the question for you is. How fast do you have to run that second lap, so that your total average speed, is two V1? Number four, any time a train is moving, either slowly or very fast. There is a part of the train which is actually moving backwards with respect to the ground, that is not backwards with respect to the train, and it doesn't have to be going so slowly that if you ran backwards you would be going backwards with respect to the ground. It is at any speed, any normal train will have a part of it, which is moving backwards with respect to the ground. My question to you is. Which part is it? now with these four riddles, you can make me a video response if you like to tell me about your solutions to them. Or you can simply leave your answers in the comments below and number them 1 2 3 4 so we know which ones you're talking about, and I will post all of the answers up and next week at the same time same place. All of my videos are made possible by my Patrons on Patreon, so if you are one of those people. Thank you so much for your ongoing support, and if you're not. Maybe now is a good time to consider becoming one, if you go to Patreon.com/Veritasium you can support me there. Otherwise, if you're interested in more riddle-y puzzling things, you can check out my Spinning tube trick, my 5 fun physics phenomena, and say the spinning district and if you haven't subscribed Veritasium already I highly recommend you do that. And since i'll be posting the answers next week I would like for you to ring the bell down below, and that will make sure that you're notified when my videos come out. So as always, thanks for watching wait that's, that's Michael's line.
I was bored so I figured I'd make an animation about riddle 4) where the outer part of the wheels rotate below the track.
This way, the part of the wheel that is below the track moves backwards with respect to the ground/track and can be visualised by tracing parts of the wheel during the rotation.
http://i.imgur.com/eqwpvNu.gifv
For #3, this video is awesome.
https://www.youtube.com/watch?v=aJhiY70KY5o
My Response to the 4 Revolutionary Riddles
You can use a viscous fluid and some weights. The "do it at home" example I see most often is honey and a few marbles.
It would remain stationary. The back wheel will eventually skid against the ground until the pedal is aligned with your string, then you can start pulling the bike (again with the back tire skidding).
We can write L=vt and L=v't' and have the condition 2L=2v(t+t') which together imply that t=t+t' which can only be true for t'=0, aka an average speed of 2v is the limit as v' approaches infinity.
I'm unsure on this one. The bases of the wheels would be stationary, but that's the closest I can think of.
EDIT: Reworded my answer for 2 to be a little more clear.
Any viscous fluid in the cylinder will do the trick
Assuming enough force is applied to overcome friction, the bike will move backwards
It's not possible because you would have to travel infinitely fast. You can't simply average the speeds together because of the different amounts of time spent at each speed
There's actually a part on the inside of the train wheels that are below the top edge of the track, which is the center of rotation for the wheels, so they would be moving backwards relative to the tracks
I'm probably wrong about everything, but this is what I got.