studiot Posted May 11, 2015 Share Posted May 11, 2015 We have had umpteen threads about forces and accelerations on rotating bodies, without result, so here is a question for those who think they know. Put a cup, half full of tea, on the outer edge of a motorised lazy susan and spin. What will happen to the surface of the tea in the cup. Will it rise up one side, if so up the inner (close to the centre) or the outer? What does this mean for the centrifugal v centripetal argument? I have tried to find a video of this experiment without success. I can only find videos showing the vortex created by spinning the cup about its own axis. So I woudl be grateful if anyone can offer one. 1 Link to comment Share on other sites More sharing options...
swansont Posted May 11, 2015 Share Posted May 11, 2015 You want something to move in a circle. Here's the wrong line of reasoning: there's an outward push. Here's the right line of reasoning: there needs to be in inward push. What is going to provide that? Link to comment Share on other sites More sharing options...
Delta1212 Posted May 11, 2015 Share Posted May 11, 2015 (edited) I'd assume it should rise up on the outer edge, as the cup is accelerating towards the center. It's effectively the same result you'd get by sliding the cup quickly across the table and having the tea slosh backward over your hand. Am I missing something about this set up that causes a non-obvious result? Edited May 11, 2015 by Delta1212 1 Link to comment Share on other sites More sharing options...
imatfaal Posted May 11, 2015 Share Posted May 11, 2015 It will rise up the outer edge 1 Link to comment Share on other sites More sharing options...
Enthalpy Posted May 11, 2015 Share Posted May 11, 2015 Is Studiot testing whether he can attract us to complicated and wrong answers just through the formulation of the question? Link to comment Share on other sites More sharing options...
Strange Posted May 11, 2015 Share Posted May 11, 2015 I'd assume it should rise up on the outer edge, as the cup is accelerating towards the center. It's effectively the same result you'd get by sliding the cup quickly across the table and having the tea slosh backward over your hand. Great example: if the cup is slid across the table, it is obvious that the force is in direction of motion. There is obviously no force in the other direction pushing the tea up the cup. Just like the circular motion example. Link to comment Share on other sites More sharing options...
imatfaal Posted May 11, 2015 Share Posted May 11, 2015 https://youtu.be/hudepr3XeuQ Don't know why that won't embed properly. www.youtu.be/hudepr3XeuQ 4 Link to comment Share on other sites More sharing options...
studiot Posted May 11, 2015 Author Share Posted May 11, 2015 Thanks for the video, Google didn't find it for me. Link to comment Share on other sites More sharing options...
imatfaal Posted May 11, 2015 Share Posted May 11, 2015 Thanks for the video, Google didn't find it for me. That is probably because I only just filmed it! It's the front wheel from my trusty hybrid, a carton which had nuts in, a bit of sellotape, some inky water, and some very bad cinematography 3 Link to comment Share on other sites More sharing options...
studiot Posted May 11, 2015 Author Share Posted May 11, 2015 (edited) I wondered about that, I didn't think of a bicycle, though I've just pumped up a tyre this afternnoon. +1 I haven't the resources to do any graphics until June. BTW I also never knew St James berries were red Edited May 11, 2015 by studiot Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted May 12, 2015 Share Posted May 12, 2015 Put a cup, half full of tea, on the outer edge of a motorised lazy susan and spin. What will happen to the surface of the tea in the cup. Will it rise up one side, if so up the inner (close to the centre) or the outer? When accelerating the spin, it can rise opposite the acceleration. This direction can become inward with the first part of the rotation, before it becomes outward...and can slosh around before becoming primarily outward. Link to comment Share on other sites More sharing options...
Robittybob1 Posted May 12, 2015 Share Posted May 12, 2015 It will rise up the outer edge Spin it fast enough it will go over the edge of the cup. Link to comment Share on other sites More sharing options...
MigL Posted May 12, 2015 Share Posted May 12, 2015 Shhhhhh... Don't tell Mike Smith about this or we'll never hear the end of it. Link to comment Share on other sites More sharing options...
studiot Posted May 12, 2015 Author Share Posted May 12, 2015 Here is an a video of a glass of liquid during an airplane roll. https://www.youtube.com/watch?feature=player_detailpage&v=uw2qPLEgKdQ#t=3 1 Link to comment Share on other sites More sharing options...
studiot Posted May 14, 2015 Author Share Posted May 14, 2015 (edited) Here is an interesting sequel to imatfaal's experiment, courtesy the University of Illinois. https://van.physics.illinois.edu/qa/listing.php?id=14308 Edited May 14, 2015 by studiot Link to comment Share on other sites More sharing options...
StringJunky Posted May 14, 2015 Share Posted May 14, 2015 Here is an interesting sequel to imatfaal's experiment, courtesy the University of Illinois. https://van.physics.illinois.edu/qa/listing.php?id=14308 The sand ends up in the middle because the vortex, made by stirring, is a funnel: descending grains follow the most active part of the funnel down. into the 'eye' at the bottom. I was thinking of tornados. Link to comment Share on other sites More sharing options...
studiot Posted May 15, 2015 Author Share Posted May 15, 2015 The interesting thing about imatfaal's video is the question How does the cup of tea get past the cycle forkhead? Link to comment Share on other sites More sharing options...
Spyman Posted May 15, 2015 Share Posted May 15, 2015 The interesting thing about imatfaal's video is the question How does the cup of tea get past the cycle forkhead? The front wheel is not mounted on the bike. Link to comment Share on other sites More sharing options...
imatfaal Posted May 15, 2015 Share Posted May 15, 2015 The front wheel is not mounted on the bike. Quite correct. Although I have ridden one of these which would have made the whole setup much easier http://www.cannondale.com/uk_gb/2015/bikes/black-inc/mountain/trigger Link to comment Share on other sites More sharing options...
kisai Posted May 15, 2015 Share Posted May 15, 2015 If the liquid in the teacup was frictionless, would the surface change? Link to comment Share on other sites More sharing options...
imatfaal Posted May 15, 2015 Share Posted May 15, 2015 If the liquid in the teacup was frictionless, would the surface change? Yes I think it would. But I think the viscosity of the liquid would matter even more - if I had filled the container with chain lube rather than water I think the angle would have been shallower than water which in turn would be shallower than ethanol (I could use vodka?). But maybe this would only really affect the time taken to reach equilibrium rather than the point of equilibrium itself. Good follow-up to a good op. I have guests in my office all of next week so no chance for a follow up experiment with other liquids I am afraid Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted May 16, 2015 Share Posted May 16, 2015 Yes I think it would. But I think the viscosity of the liquid would matter even more - if I had filled the container with chain lube rather than water I think the angle would have been shallower than water which in turn would be shallower than ethanol (I could use vodka?). But maybe this would only really affect the time taken to reach equilibrium rather than the point of equilibrium itself. Good follow-up to a good op. I have guests in my office all of next week so no chance for a follow up experiment with other liquids I am afraid If the liquid is frictionless, isn't the viscosity assumed to be negated? Link to comment Share on other sites More sharing options...
imatfaal Posted May 16, 2015 Share Posted May 16, 2015 If the liquid is frictionless, isn't the viscosity assumed to be negated? I don't know - I think you are right. The only frictionless liquid I could think of was supercooled liquid helium - and I wasn't even totally sure of what a frictionless liquid is other than superfluidity. So what I meant to imply by my post was that levels of room temperature viscosity was a more important and approachable question Link to comment Share on other sites More sharing options...
Travis Hallet Posted May 18, 2015 Share Posted May 18, 2015 I didn't take the time to see if any one said it but I've got to Do it. Murphy's Law says the tea will spill in the direction of you. Link to comment Share on other sites More sharing options...
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