Friday, November 19, 2010

On Falling Food, Part I: Buttered-Side Down

Earlier this week my little sister, Ashley, asked me to explain why a dropped piece of bread usually lands buttered-side down. I assume that she was referring to an example of Murphy's Law (or Sod's Law) in action. Namely: "the probability that a piece of buttered [1] bread will land buttered-side down is proportional to how expensive the carpet is."[2] But Ashley didn't include the correlation with the price of the carpet, so I'll just address the question, "why does a dropped piece of bread usually land buttered-side down?" The short answer is: it doesn't.

Now, I'm sure she won't be satisfied with the short answer. So here's some more detail. In 2005 Mythbusters tackled this idea in one of their shows (Season 3, Episode 28 "Is Yawning Contagious?"). They tested two different conditions. First they dropped buttered toast by sliding it off a table. It usually landed buttered-side down. Second, they dropped buttered toast off of a building. It was a 50-50 split whether it landed on one side or the other.[3] Now this may be nitpicky, but Ashley's original query didn't say anything about the height that the toast is dropped from. So without height as a confounding variable, buttered bread, when dropped, is equally likely to land with either side down.

The idea has been put forth that bread lands buttered-side down because the butter makes that side heavier. While that is true, that has no bearing on how fast one side or the other falls. Recall that during the Apollo 15 visit to the moon, astronaut David Scott dropped a hammer and a feather—and they fell at the same rate.[4] The only reason things fall at different rates on the Earth is because of air resistance. A piece of bread with butter on it has nearly the same air resistance as a piece of bread with no butter. So the extra weight of the butter only has bearing in a single instance: when the bread lands on it's edge—then the bread is more likely to tip to the buttered side.[5]

So why does bread pushed off a table land more often buttered-side down? As it slides over the edge of the table (or slips out of your hand), one side of the bread starts to fall while the other side is still supported by the table or your hand. This introduces a spin momentum to the bread.[6] After that it's simply fate that the height of a human being's hand (or their table) is only enough for the bread to complete half a turn (180°) before it hits the floor. If we were ten to twelve feet tall and had tables and counters that were about six feet tall, then this phenomenon would be completely unknown to us.



Notes:

[1] Other versions have jam or jelly, which are more likely to stain the carpet.

[2] Another example (which I've experienced more than my fair share of) is: "On the days you leave your car windows down, it will rain; on the days you leave the windows up, it will be unusually hot."


[4] If you were unaware of this experiment, you can watch it here, on YouTube.

[5] But even if it falls on its edge and then tips to the non-buttered side, you're still likely to end up with some butter on your carpet from the initial impact.

[6] This can be compounded by fruitless attempts to catch the falling bread.

Image attribution:

Breads and spreads is by Velvet Android, available at http://www.flickr.com/photos/68777870@N00/498984957.

1 comment:

  1. I am anxiously awaiting the five second rule post.

    ReplyDelete