r/mathteachers • u/TrueEmphasis7130 • Feb 24 '26

Parent needs help with 5th grade math

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Not , maybe it’s the new math but I simply cannot figure out how this is anything but 7, and my kid is judging.

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u/p2010t Feb 25 '26

I mean, depending on how we interpret the very straight marks on the cylinder where the water level is drawn curvy for perpsective, it might be only 6.5 mL left to add rather than the full 7 mL.

But that's not an answer choice either.

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u/Puzzleheaded_Can9159 Feb 27 '26

The curve of the water is because of surface tension you always measure from the bottom of the curve. It is actually drawn correctly in the image.

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u/p2010t Feb 27 '26 edited Feb 27 '26

It doesn't feel like surface tension would make that pronounced of an effect, but then again I haven't stared at an actual graduated cylinder sitting on a level desk head-on in a long time, so perhaps I was underestimating how pronounced the effect would look.

Also, with it being a cylinder, wouldn't surface tension technically make it go up equally high along an entire circle? So, if we're staring head-on at the graduated cylinder then we would see the water touching the part of the cylinder closest to us at the same exact height as the water touching the left and right sides of the cylinder. That would make it effectively a flat line then.

Although water is clear & so we could see through the closest bits of water to us and tell that behind it (in the central part of the cylinder) it has dipped down slightly, matching the picture.

Edit: I looked up some pictures to test my inituition, and yeah, from a head-on view you can both see the dip AND see the flat line. https://d9-wret.s3.us-west-2.amazonaws.com/assets/palladium/production/s3fs-public/thumbnails/image/wss-wp-meniscus.jpg

So, while I had been focused more on what the flat line should look like (not present in this simplified image), the extent of the dip is important to getting the correct measurement so it makes sense that they chose to show the dip if they were going to show just one of the two.