4 Revolutionary Riddles
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 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. 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. 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 trying 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 that 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. I will post all of the answers up 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. 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.
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