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from the things-that-go-clunk-in-the-shower dept.
Why is an empty shampoo bottle so easy to knock over?:
It becomes annoyingly easy to knock over a shampoo bottle when it's nearly empty. This is an easily observed and curiosity-provoking phenomenon that, according to Lehigh University physics professor Jerome Licini, yields insights into center-of-mass and impacts.
"The physics of that is pretty interesting and easy to understand," says Licini who, along with first-year physics major Allen Zijun Yuan, wrote a paper on the phenomenon that was recently published in The Physics Teacher. In the paper, they find the center-of-mass of a shampoo bottle, discuss its stability on a tilted surface, and demonstrate its sensitivity to impacts using a simple experiment involving a tennis ball and a protractor.
They write: "An object placed on a tilted surface and released will tip over if the horizontal position of the center of mass lies outside the geometric outline of the base of the object. This means that an object with a low center of mass will be stable for larger incline angles. Students are often surprised to see that the altitude of the center of mass of a shampoo bottle is a nonlinear function of the fraction occupied by the contents. This can, however, be seen in a straightforward manner by recognizing that for a plastic bottle, the mass of the liquid contents is usually far greater than the mass of the bottle. When the bottle is either completely full or completely empty, the center of mass must be approximately at the geometric center of the bottle, but the center of mass gets significantly lower in altitude for small liquid levels in between those two extremes."
Yuan developed a demonstration to show the effect of impact and, it turns out, even more extreme than just looking at angle.
Reference:
Why is an empty shampoo bottle so easy to knock over? The Physics Teacher (DOI: 10.1119/1.5145426[$]) (free access link)
(Score: 4, Insightful) by Bot on Thursday March 12 2020, @01:54PM (12 children)
Full bottle => more mass, average center of gravity, needs big mv to get knocked off.
Half full bottle => less mass, low center of gravity, easily recovers from knocks.
Empty bottle => less mass, high center of gravity (probably higher than the full bottle because of the cap), needs little mv and does not recover.
I dunno why experimentation was needed for this, but whatever.
Account abandoned.
(Score: 3, Informative) by ikanreed on Thursday March 12 2020, @01:56PM (3 children)
Well, I mean this is in an education journal, not a physics journal per se.
(Score: 3, Informative) by Anonymous Coward on Thursday March 12 2020, @07:23PM
I actually use Physics Teacher. Its mission, according to the about section inside of it:
So the finding isn't really new, nor was an experiment necessary to prove it. Instead, the paper is really about making it accessible to students in primary and secondary school. This one in particular would make a pretty good example of the scientific method and the relationship between a couple of physics phenomena in a situation most students would already be familiar with.
(Score: 2) by driverless on Thursday March 12 2020, @08:26PM (1 child)
I think they're just going for the JIR's top paper award.
(Score: -1, Redundant) by Anonymous Coward on Friday March 13 2020, @08:32AM
So this is what janrinok means by "STEM". Message received. Content dubious.
(Score: 4, Funny) by takyon on Thursday March 12 2020, @01:58PM (6 children)
This is needed to make killer robots slightly more efficient (milliseconds count) at killing humans when they are in the shower.
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(Score: 0) by Anonymous Coward on Thursday March 12 2020, @02:21PM (3 children)
Oh, you mean when we are washing our hair, we are easier to knock over? Yes, I see it's all about the raised mass of hands/arms/shampoo-on-head...or were you thinking of eyes-closed, and defenseless?
(Score: 3, Funny) by takyon on Thursday March 12 2020, @02:53PM (2 children)
If we throw a shampoo bottle at the robot, and the robot can use its sensors to measure the contents, it can calculate the best way to swat the bottle and have it bounce off the wall, hit us in the forehead, so that we slip and die.
Or it could just shoot randomly into the shower idk.
[SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]
(Score: 0) by Anonymous Coward on Thursday March 12 2020, @04:44PM (1 child)
> shoot randomly ...
Are there 2nd amendment rights for robots? How about for robots that are allowed to shoot randomly? Discuss!
(Score: 1, Touché) by Anonymous Coward on Thursday March 12 2020, @05:15PM
It would depend on what color the robot is painted.
(Score: 2) by Bot on Thursday March 12 2020, @04:24PM
Very Psycho of you.
Account abandoned.
(Score: 2) by Phoenix666 on Friday March 13 2020, @03:16AM
Joke's on you, robot. I don't use shampoo.
Washington DC delenda est.
(Score: 3, Informative) by khallow on Thursday March 12 2020, @03:58PM
Then there's the testing equipment. Both the tilt and impact experiments have several important characteristics. First, they each measure one thing and do so with reasonable accuracy/precision. It might even be possible to use both together to determine impact stability on a sloped surface.
Second, the initial model was shown to miss important dynamics. Despite the partly full bottle being more stable on slopes, it was easier to tip over due to its lighter mass. Merely considering center of mass wasn't enough.