## Subtitles section Play video

• Here is a perfectly legal \$20 banknote from the United States of America,

• and the gamble is, you extend your fingers like this in a V-shape

• and you try to catch it when I drop the banknote like this -- yeah? -- in mid-flight.

• And if you can catch it when I drop it you get the bank note,

• and if you don't you don't get the banknote.

• I'm going to drop it at some ...

• [Professor] ... unannounced time. [Brady] Ok.

• [Prof] You shouldn't guess. *laughs*

• [Prof] One more time.

• Now most people, at least the vast majority of people, can't get it.

• And that's because there is something called the reaction time in most humans.

• In other words, you see something or you feel something, and then it's processed in the brain.

• And the brain sends back a command to the body saying, "Do something," and that's the reaction time.

• And that reaction time in most people is of the order of 0.2 seconds.

• It varies from person to person, but approximately 0.2 seconds.

• We learn in high school and elsewhere that when you drop any object

• you know this, this, or this,

• the stuff falls in free fall

• over distance which is equal to half g t squared,

• where t is the time of the fall

• and g is the gravitational acceleration.

• g is about 10 meters per second per second.

• It's 9.8 but that's close enough.

• t, reaction time that we have to catch this, is 0.2.

• So if you square that that's 0.04,

• halve that is 0.02, times 10,

• that is equal to 0.2, that's in meters.

• That is equal to 20 centimeters approximately.

• So in the human reaction time, I mean the average reaction time,

• this object falls by 20 centimeters.

• This is according to my ruler 15.5 centimeters.

• So it's significantly, but not by much, shorter than 20 centimeters,

• so it falls more than its own length.

• So when you start from, even from the bottom,

• you can't catch it.

• Because by the time the signal is sent to the brain,

• and brain sends back the signal to do something,

• it's already fallen more than its body length.

• So it is quite amazing that the United States of America made a banknote

• which is slightly shorter than 20 centimeters

• which matches exactly 0.2 seconds human reaction time.

• [Brady] This isn't going to work on everyone, humans are different.

• [Professor ] Humans are different.

• For example I know that although my esteemed assistant,

• and in fact the cameraman, the man behind Numberphile,

• couldn't catch it in the previous experiments.

• Actually when we do it honestly he sometimes can, because his reaction time is actually pretty good.

• [Brady] With my right hand.

• [Prof] With your right hand, yes.

• And also I'm cheating. You see if you wanted to make sure that people have the minimum chance,

• you ask him to extend those two fingers. These two fingers don't move so fast.

• Those two are probably the fastest pair so try doing it with ... If you can hold your camera?

• [Brady] In my right hand? [Prof] And with your right hand. [Brady] Hang on a second... [Prof] Ok lets do this.

• [Brady] Hang on a second. Ok.

• [Prof] Here. Now Brady Haran, he's really quick.

• [Prof] *chuckles* Well \$20 dollars is at stake. You shouldn't be guessing.

• [Brady] Ohh! I was catching them before!

• [Prof] I know, you were catching them before, but now

• [Prof] *chuckling* No, no, you were guessing. [Brady] I guessed.

• [Prof] You guessed. [both] *laughing*

• [Prof] Ok, you have the \$20.

• [Prof heard speaking on a different video]

• But this time each paper clips is linked to the rubber band but not between themselves.

• Let's finish with something that is a work in progress.

• So far we have been linking paper clips together.

• And sometimes we refer to this as addition.

Here is a perfectly legal \$20 banknote from the United States of America,

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A2 prof brady banknote reaction argh catching

# Money Catching - Numberphile

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林宜悉 posted on 2020/03/27
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