# Is is possible to design a cup lid that doesn't alter the trajectory of the cup when thrown?

First, some context to my question. Second, I do not have a background in fluid dynamics/engineering. Liberal Arts major FTW.

I've been running observational experiments with my 14 year old to demonstrate how fluid dynamics work in real life. Our experiment began as follows: I took a 16 oz plastic cup, filled it with 4oz of water, grabbed the rim of the cup between my thumb and forefinger, and threw the cup overhand with a tomahawk motion. The cup's trajectory is a consistent arc with the bottom of the cup facing down/forward. A little bit of water comes out of the back/open side during the arc.

We add a plastic lid to the cup. This time, after release, the water inside the cup sloshes back and hits the lid, causing the cup to rotate vertically 1-2 revolutions.

So here's my son's question, which I cannot for the life of me pull off in home prototyping - Is it possible to design a vessel (doesn't need to literally be a cup, just something approximately 16oz in volume) that enables you to "cap" the vessel so the water doesn't escape then thrown, but also doesn't interfere with a smooth trajectory/the cup doesn't tumble.

We researched this question here on Physics.SE and this is the most relevant thread. It discussed why the trajectory is impacted, but doesn't discuss designs that wouldn't prevent a smooth trajectory.

• Why not a vertical walled vessel with a screw on lid that makes it into a uniform cylinder? Though as it is this is really more of an engineering question than a physics one. Commented Apr 16 at 15:07
• That's something we've been playing with - various sized bottled water containers that generate that kind of effect. We haven't found one that flies straight where the bottom of the container (the liquid ballast end of it) consistently faces forward/down. Commented Apr 16 at 17:15

## 1 Answer

If you put any cup at a string and throw it around fast enough in a circle, so the opening is down the liquid stays in the cup. If you manage to throw it in a similar circle the same will happen.

• I appreciate the suggestion - we'll experiment with the method of the throw / minimize the sloshing effect Commented Apr 16 at 17:21