I recently saw this video of a young woman balancing her entire weight on some glasses. Before having seen it, if you had asked me if even a 90lb woman could balance on two glasses without smashing them I would have told you categorically that it was not possible. I was clearly wrong. Can anyone explain the basic physics that allows her to do this (somewhat) safely?
Do we know those glasses are glass rather than plastic? I’m not saying it couldn’t be done on glass glasses, but I suspect plastic would be a lot safer. If they are glass, the thickness of the glass is gong to matter a lot. Some glasses are pretty sturdy.
Because the direction of the force is important.
OldGuy: At 0:07 the glasses clink together and make a distinctive glass noise.
Glass can actually be a very strong material. Especially in the direction she’s putting the force on it. Notice that she’s very carefully keeping the force from her hands straight down on the glass. And she looks pretty tiny, so what, maybe 50-60 lb on each glass? Presumably she tested the strength of the glasses ahead of time as well.
Well I’ve been meaning to get speakers for this computer for some time. – Actually I haven’t. I much prefer my computer to be silent and keep the volume off.
It’s a little risky with those glasses because they aren’t straight sided. The shape does seem to concentrate the load back on to straight portions of the glass. Glass has low tensile strength but very high compressive strength and as long as the weight is primarily concentrated in a vertical direction they will not easily break. Consider a brick building, the bricks have high compressive strength allowing them to hold up enormous weight. It does not take a lot force on a brick wall in a horizontal direction to make that building crumble.
A colleague of mine was once asked to use his classic Bentley in a commercial to demonstrate the strength of some ceramic teacups for a marketing promotion. They jacked up his car and placed one teacup under each wheel and then gently lowered the car down onto the teacups. Within seconds the cups shattered. Assured that the cups would hold up the weight they tried it twice more with the same results. Contacting the teacup manufacturer they found that they had the wrong cups. The ones they used had a slight flare at the rim which directed the force outward and they could not hold up the weight. Once the proper cups which were basically straight sided were used the cups held up the car without a problem.
It’s easy to demonstrate this for yourself. Take a paper towel tube and stand it up vertically and push downward on it. It will take a lot of force to make it crumple. Turn the tube so that it is horizontal and just push down lightly with one finger and it will fold in on itself.
AKA buckling failure Buckling - Wikipedia
A nasty failure mode where strength suddenly collapses to a small fraction of what it normally is/was.
Yes it’s the direction of the force that matters… and actually a small woman’s weight being supported by 2 glasses isn’t that impressive. I can (carefully) stand on an empty pop can without it crushing. If the can’s edges don’t have any dents you can load it straight up and down with at least 200lbs and it won’t crush; think of the difference in thickness between the glass and the metal sides of a can.
Here is some calcs for perspective. Assume cup outside diameter is 3.5 inches (8.9 cm) and the thickness of the glass is 0.1 inches (0.25 cm). Then the cross sectional area is approx 1 sq inch or 689 sq mm or 6.9 sq cm.
The compressive strength is 1000 N/mm2
So this cup in pure compression can withstand
689,000 Newtons or 70 metric tons or about 77 Short tons (US) - Anymore and it will break/shatter
My mind is blown. Had no idea that a car or elephant could be placed on a couple glasses without them breaking.
Well, the key is to keep the forces in compression. You can safely lower a car onto glasses or tea cups fairly easily. An elephant might want to shift his foot a bit sideways at some point though, and the lateral forces will easily shatter the glass.
Similarly, turn the glass on its side and it becomes very weak.
The girl doing the handstand has to be very careful to keep herself very well balanced. The glasses can hold her weight (plus quite a bit more) as long as the forces are all in compression. If she moves side to side to steady herself though, too much movement can put too much lateral force on the glass and the glasses will shatter.
I see how you calculated that but in the link it talks about a “cube of glass.” Doesn’t the shape matter and not just the cross-sectional area?
If you want to see some really interesting behavior of glass under compression and tension, look up Prince Rupert’s drops. You can whack it with a hammer all day and not break it, but just break the tail and it explodes. The linked video has some spectacular slow-motion shots of a drop breaking.
Like the video of the handstand, it’s one of those “don’t try this at home, kids” sort of thing. Well, if you wear safety glasses, it’s probably safer, because there’s no failure scenario for the young lady in the handstand video that doesn’t end with bloodstains everywhere if a glass does break.
It helps that she is circus people.
This is why I always crimp the side of an aluminum can before putting it into the hand crusher. If you don’t, you can really torque your wrist when the can doesn’t crumple like you expect it to.
Yeah. Gravity just doesn’t apply to those folks like it does the rest of us. Magic they are. Or so it seems. 
Yes, depending on the exactly shape you can get different kinds of stresses within an object. That’s not guaranteed however. In this case it seems the glass is almost completely in compression.
Someone mentioned a flared cup earlier. In that case the flared section has a tensile hoop stress, which is not good for ceramics.
Yeah CookingWithGas, the shape does matter. I am an engineer and love order of magnitude calculations. The objective of the calc was to show the person’s weight is far far below what is required to shatter the glass - so I assumed a cylindrical glass with a pure compression force.
I’m not super strong (or probably even sorta strong) but I can crush an empty soda can with my bare hands easier than I could a paper cup if I crimp the can 4 ways before crushing it. I just pinch the can between forefinger and thumb twice so that there are 4 evenly-spaced dimples, then I just squeeze the can between my hands top-to-bottom and it neatly crumples to a third-fourth of its height. It’s so fast and easy I can do a whole pile of cans in a few minutes and my hands are barely sore.
Try it with an unpinched can and it takes a lot of effort.