Why Do Café Tables Wobble? (Structures 6-5)
Вставка
- Опубліковано 21 сер 2021
- This might be familiar to you…sitting at a café table, you realize it rocks back and forth and stuff a bunch of paper napkins under one leg to stop it rocking. If so, then it was a table with four legs.
So, why does this happen? Because, structurally, four legs is one leg more than the minimum needed to support a plane (the tabletop).
Taking things down one dimension level, three supports is one support more than the minimum needed for a beam. With two supports I can tell you immediately how much load they’re carrying. We call this set-up structurally determinate. If we add a third support then I can’t tell you the load at the supports without knowing more about the stiffness of the beam and the supports themselves. We call this structurally indeterminate. Both have their place in designs and both have their advantages and disadvantages, including beyond just structural thinking.
But back to the humble café table, I’m always surprised so many (though not all) come with four legs. You always see people putting cameras on top of tripods because tripods don’t wobble. Ahh, if only structural engineers designed all the café tables in the world…think of all the table legs and paper napkins we’d save!
I'm Paul Kassabian. I'm a structural engineer and a Principal at SGH in Boston, MA. I taught graduate students at MIT for nine years and currently teach on/off at Harvard's Graduate School of Design (GSD). These are videos based on my years of teaching structures to students.
A four legged table/chair often doesn't provide any amount of redundancy. If a chair leg breaks you're probably going to end up on the floor.
This makes me think of the video were someone kicks the leg of a lawn chair with someone sitting it in, the leg breaks away but because there's still the 3 legs on the ground the chair doesn't move making the person kicking look alittle silly.
Great content. What won the subscribe, though, was the adorable camera operator and the fact you are involving her in your work. 👍
Great camerawork! 👏👏
Great series. Thanks. Trick for stabilizing a 4 legged table - rotate it about its vertical axis and it will transition through a position where all four legs contact the uneven ground surface and stop wobblling. Of course, this has nothing to do with the original premise of the video, but a good parlor trick anyway.
You're welcome and, yes, good note on the table solution...slightly trickier of course to do that with a building...
I've read that you don't need to turn the table more than a quarter of a turn to find a place where the table no longer wobbles.
So appropriate that we have a wobbly camera person for a video about wobbly tables. I was getting a little motion sickness for a moment there.
Hah - unintentional but it sort of worked out!
Beautiful explanation! PS Congratulations to your daughters
Thank you!
Hi Paul, thank you for these videos! I can't wait to learn about frames.
Ta-dah...ua-cam.com/video/CRvzojhAyg8/v-deo.html
Great video. I learned it this way: Any 2 points are co-linear. Any 3 points are co-planear.
That is how I learnt it in 2010 when I was applying for med school, during geometry class. It always stuck with me and today, as a woodworker, I end using this knowledge time and time again.
Paul, you're going to need another cup of coffee🤣
PS: Your videos are fascinating.
Thanks!
Very illustrative. Suddenly structural engineering is simplified. Thanks a lot!
You are welcome!
Just before the end, there was evidence that there was real coffee inside the coffee cup that spilt when the table rocked ever so slightly but the fluid took a whole dynamics of its own.. Anyone explain and link the fluid dynamics here, would be a nice one to watch and learn. Cheers!
Edit: BTW, a golden series coming up here on structural engineering. Kudos to you Kassabian! Many many thanks.
thanks for the note!
good work summing up some of the most fundamental concepts of structural eng with the perfect example
Thanks!
You missed a great opportunity to introduce the Wobbly Table Theorem! It's practical application is that it can actually be used to save napkins.
Last giggle made me come back to reality..
your lectures are eyeopenrs for bookish architects।।।
hey paul, enjoyed your truss video. I am trying to make a carport out of steel tube similar to the one april wilkerson made on her YT channel. Hoping to make it look timber frame-like, I am considering a king post vs queen post truss configuration. A fan truss looks too modern in my opinion. In researching residential frame construction, I saw a QP generally would span greater distances than a KP, which I guess means it is more effective at resisting loads. If you get a chance, seeing an engineering breakdown of these more historical building solutions would be cool. Thanks for your educational efforts. You have a new subscriber
Dear paul !can you make a video on how we can see and tell what members of a structure are in tension and compression . A bit of explaining both these forces actually.
This is a great video, and worth the tea sacrifice!
I have a question that might be fun to answer in a video. If you have a continuous beam with three supports, so two ends supported and a central support, is that still treated as statically indeterminate in design? At what point when you are moving the central support away from the middle does it become so? Or what difference in support height? How do engineers get away with generally treating the supports for such a beam as rigid and on a flat plane with known reactions and bending moments?
Hahahah. I am loving this series.
Now that we are talking about tables, I recently came across an interesting table design called "tensegrity table". It blew my mind away. I am now looking for a local woodworker to make one for me.
Great explanation sir. Please do more videos. Love from India 🤩
Thank you, more to come!
Please do lecturers on structural analysis part
Linked to this - Mathematically you can rotate the table to find a point where all 4 legs touch the ground - there's a proof of such by means of the intermediate value theorem. It's based on a few assumptions and kinda doesn't work in practice, unless the floor is smooth. ua-cam.com/video/OuF-WB7mD6k/v-deo.html
What a great video! Thanks for sharing. I'm still going to advocate for 3-legged tables...it means fewer friends, less tabletop, less fun...but structurally stable!
The professor suggested a way to find a steady spot for a table with four legs using a simple illustration. Is it guaranteed that we can always find four flat points on an uneven floor in real-life situations?
My favourite kind of English accent - Ed-dew-cated.
Not once did he utter, "let me arx you sumfin, yeah?"