# Etiqueta: Physics

## bowling balls

Got to see this demonstration of physics by bowling balls this weekend. It’s beautiful and cool. Watch all the way to the end to see the balls move back in phase with each other, as they are at the start. In between, it moves from beautifully ordered to apparent chaos and back, again and again. […]

Got to see this demonstration of physics by bowling balls this weekend. It’s beautiful and cool. Watch all the way to the end to see the balls move back in phase with each other, as they are at the start. In between, it moves from beautifully ordered to apparent chaos and back, again and again.

Because this video has been very popular, here are answers to some common questions:

** What am I seeing? How does this work? **

The length of time it takes a ball to swing back and forth one time to return to its starting position is dependent on the length of the pendulum, not the mass of the ball. A longer pendulum will take longer to complete one cycle than a shorter pendulum. The lengths of the pendula in this demonstration are all different and were calculated so that in about 2:40, the balls all return to the same position at the same time – in that 2:40, the longest pendulum (in front) will oscillate (or go back and forth) 50 times, the next will oscillate 51 times, and on to the last of the 16 pendula which will oscillate 65 times. Try counting how many times the ball in front swings back and forth in the time it takes the balls to line up again, and then count how many times the ball in back swings back and forth in the same time (though it’s much harder to keep your eye on the ball in back!).

** Why are they not perfect at the end? **

This large frame is built from wood and is outdoors, which means it expands, contracts, and flexes. Because the position of the frame changes, the cycle lengths are not perfectly aligned. Over time, the minor differences become more pronounced.

** Where is this? **

This was built on private property in the mountains of North Carolina (United States), near Burnsville. It is not open to the public.

** Can I get a copy of this video to use in my classroom? **

Yes. But I don’t have a link for you yet. Come back in a day or so to get the link here.

** How can I make my own? Where can I learn more? **

Here are some links to information about the physics behind this demonstration. There are some small scale versions of this demonstration that can be purchased commercially as well, but if you want a 20’ version like this, you’ll have to make your own! I didn’t make this and I don’t have plans for it, but work through the physics at these links and design your own – you’ll learn a lot about physics, math, and construction!

– http://www.arborsci.com/cool/pendulum-wave-seems-like-magic-but-its-physics

— http://io9.com/5825639/a-simple-physics-demonstration-that-shows-why-science-still-sometimes-seems-like-magic

— http://sciencedemonstrations.fas.harvard.edu/icb/icb.do?keyword=k16940&pageid=icb.page80863&pageContentId=icb.pagecontent341734&state=maximize&view=view.do&viewParam_name=indepth.html#a_icb_pagecontent341734

–http://scitation.aip.org/content/aapt/journal/ajp/69/7/10.1119/1.1349543?ver=pdfcov

**Published on Mar 8, 2012**

A pendulum wave with 16 bowling ball pendulums of different lengths, Doc’s Pasture, Celo NC. Another good view with people lying undeneath: http://youtu.be/RNxqEudZ-hE The math behind this and some more details are at: http://celophoto.blogspot.com/2012/03…

## 2D Rubens’ Tube

## the academic boycott of Israel

Harriet Sherwood and Matthew Kalman in Jerusalem The Guardian, Tuesday 7 May 2013 Professor Stephen Hawking is backing the academic boycott of Israel by pulling out of a conference hosted by Israeli president Shimon Peres in Jerusalem as a protest … Continue reading →

Harriet Sherwood and Matthew Kalman in Jerusalem

The Guardian, Tuesday 7 May 2013

Professor Stephen Hawking is backing the academic boycott of Israel by pulling out of a conference hosted by Israeli president Shimon Peres in Jerusalem as a protest at Israel’s treatment of Palestinians.

Hawking, 71, the world-renowned theoretical physicist and former Lucasian Professor of Mathematics at the University of Cambridge, had accepted an invitation to headline the fifth annual president’s conference, Facing Tomorrow, in June, which features major international personalities, attracts thousands of participants and this year will celebrate Peres’s 90th birthday.

Hawking is in very poor health, but last week he wrote a brief letter to the Israeli president to say he had changed his mind. He has not announced his decision publicly, but a statement published by the British Committee for the Universities of Palestine with Hawking’s approval described it as “his independent decision to respect the boycott, based upon his knowledge of Palestine, and on the unanimous advice of his own academic contacts there”.

## Whipcracking

Whipcracking is the act of producing a cracking sound through the use of a whip. Originating during mustering and horse driving/riding, it has become an art of its own. A rhythmic whipcracking belongs to the traditional culture among various Germanic peoples of Bavaria(Goaßlschnalzen), various Alpine areas (Aperschnalzen), Austria, and Hungary (Ostorozás). Today it is a performing art, a part of rodeoshow in United States, a competitive sport in Australia and increasingly popular in the United Kingdom, […]

**Whipcracking** is the act of producing a cracking sound through the use of a whip. Originating during mustering and horse driving/riding, it has become an art of its own. A rhythmic whipcracking belongs to the traditional culture among various Germanic peoples of Bavaria(*Goaßlschnalzen*), various Alpine areas (*Aperschnalzen*), Austria, and Hungary (Ostorozás). Today it is a performing art, a part of rodeoshow in United States, a competitive sport in Australia and increasingly popular in the United Kingdom, where it crosses boundaries of sport, hobby and performance.

## food on the floor

Published on Nov 30, 2012 Watch me lift a car (and then drop it) with phone books!http://bit.ly/Tx6cd2 Vsauce video on “touch” and why bananas are radioactive:http://bit.ly/YfocQL Vsauce on THNKR (4-part interview with me!): http://bit.ly/Twsah2 http://www.twitter.com/tweetsauce http://www.facebook.com/vsaucegaming All music by http://www.youtube.com/JakeChudnow 5-second rule facts: http://www.snopes.com/food/tainted/dr… http://science.howstuffworks.com/scie… http://en.wikipedia.org/wiki/Five-sec… Applied Microbiology: http://onlinelibrary.wiley.com/store/… Jillian Clarke: http://www.howard.edu/newsroom/releas… Mythbusters: http://dsc.discovery.com/tv-shows/myt… minutephysics on “touch”: http://www.youtube.com/watch?v=BksyMW… Electrons as waves: http://www.rhythmodynamics.com/Gabrie… […]

## limits

## yellow

## Superman

Published on Jun 13, 2013 Gain more muscle with AsapSCIENCE: http://bit.ly/11ikJxw Subscribe! It’s Free: http://bit.ly/15NrBqa *** CLICK “SHOW MORE” FOR LINKS *** DC Superman Wiki: http://goo.gl/qBltO Wolfram Alpha is the greatest tool on the internet:http://goo.gl/nk1pX Largest Nuclear Bomb: http://goo.gl/P85QC Article on how the brain puts together an image:http://goo.gl/FPGjW Amazing photos of the initial nuclear explosion:http://goo.gl/KoMdU Archive footage of nuclear […]

## Monty Hall problem

Bertrand’s box paradox is a classic paradox of elementary probability theory. It was first posed by Joseph Bertrand in his Calcul des probabilités, published in 1889. There are three boxes: a box containing two gold coins, a box containing two silver coins, a box containing one gold coin and one silver coin. After choosing a box at random and withdrawing one coin […]

**Bertrand’s box paradox** is a classic paradox of elementary probability theory. It was first posed by Joseph Bertrand in his *Calcul des probabilités*, published in 1889.

There are three boxes:

- a box containing two gold coins,
- a box containing two silver coins,
- a box containing one gold coin and one silver coin.

After choosing a box at random and withdrawing one coin at random, if that happens to be a gold coin, it may seem that the probability that the remaining coin is gold is ^{1}?_{2}; in fact, the probability is actually ^{2}?_{3}. Two problems that are very similar are the Monty Hall problem and the Three Prisoners problem.

These simple but slightly counterintuitive puzzles are used as a standard example in teaching probability theory. Their solution illustrates some basic principles, including theKolmogorov axioms.

The **Monty Hall problem** is a brain teaser, in the form of a probability puzzle (Gruber, Krauss and others), loosely based on the American television game show *Let’s Make a Deal* and named after its original host, Monty Hall. The problem was originally posed in a letter by Steve Selvin to the *American Statistician* in 1975 (Selvin 1975a), (Selvin 1975b). It became famous as a question from a reader’s letter quoted inMarilyn vos Savant‘s “Ask Marilyn” column in *Parade* magazine in 1990 (vos Savant 1990a):

Suppose you’re on a game show, and you’re given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what’s behind the doors, opens another door, say No. 3, which has a goat. He then says to you, “Do you want to pick door No. 2?” Is it to your advantage to switch your choice?

Vos Savant’s response was that the contestant should switch to the other door. (vos Savant 1990a)

The argument relies on assumptions, explicit in extended solution descriptions given by Selvin (1975b) and by vos Savant (1991a), that the host always opens a different door from the door chosen by the player and always reveals a goat by this action—because he knows where the car is hidden. Leonard Mlodinow stated: “The Monty Hall problem is hard to grasp, because unless you think about it carefully, the role of the host goes unappreciated.” (Mlodinow 2008) It is also assumed that the contestant prefers to win a car, rather than a goat.

Contestants who switch have a 2/3 chance of winning the car, while contestants who stick to their choice have only a 1/3 chance. One explanation notices that 2/3 of the time, the initial choice of the player is a door hiding a goat. The host is then forced to open the other goat door, and the remaining one must, therefore, hide the car. “Switching” only fails to give the car when the player picks the “right” door to begin with, which only has a 1/3 chance.

Many readers of vos Savant’s column refused to believe switching is beneficial despite her explanation. After the problem appeared in *Parade*, approximately 10,000 readers, including nearly 1,000 with PhDs, wrote to the magazine, most of them claiming vos Savant was wrong (Tierney 1991). Even when given explanations, simulations, and formal mathematical proofs, many people still do not accept that switching is the best strategy (vos Savant 1991a). Paul Erd?s, one of the most prolific mathematicians in history, remained unconvinced until he was shown a computer simulation confirming the predicted result (Vazsonyi 1999).

The Monty Hall problem has attracted academic interest from the surprising result and simple formulation. Variations of the Monty Hall problem are made by changing the implied assumptions and can create drastically different consequences. For one variation, if Monty only offers the contestant a chance to switch when the contestant initially chose the door hiding the car, then the contestant should *never* switch. For another variation, if Monty opens another door randomly and happens to reveal a goat, then *it makes no difference* (Rosenthal, 2005a), (Rosenthal, 2005b).

The problem is a paradox of the *veridical* type, because the correct result (you should switch doors) is so counterintuitive it can seem absurd, but is nevertheless demonstrably true. The Monty Hall problem is mathematically closely related to the earlier Three Prisoners problem and to the much older Bertrand’s box paradox.

The problem continues to attract the attention of cognitive psychologists. The typical behaviour of the majority, i.e., not switching, may be explained by phenomena known in the psychological literature as: 1) the endowment effect (Kahneman et al., 1991); people tend to overvalue the winning probability of the already chosen – already “owned” – door; 2) the status quo bias (Samuelson and Zeckhauser, 1988); people prefer to stick with the choice of door they have already made; 3) the errors of omission vs. errors of commission effect (Gilovich et al., 1995); all else considered equal, people prefer that any errors that they are responsible for to have occurred through ‘omission’ of taking action rather than through having taken an explicit action that later becomes known to have been erroneous. Experimental evidence confirms that these are plausible explanations which do not depend on probability intuition (Kaivanto et al., 2014; Morone and Fiore, 2007).

### Criticism of the simple solutions

As already remarked, most sources in the field of probability, including many introductory probability textbooks, solve the problem by showing the conditional probabilities the car is behind door 1 and door 2 are 1/3 and 2/3 (not 1/2 and 1/2) given the contestant initially picks door 1 and the host opens door 3; various ways to derive and understand this result were given in the previous subsections. Among these sources are several that explicitly criticize the popularly presented “simple” solutions, saying these solutions are “correct but … shaky” (Rosenthal 2005a), or do not “address the problem posed” (Gillman 1992), or are “incomplete” (Lucas et al. 2009), or are “unconvincing and misleading” (Eisenhauer 2001) or are (most bluntly) “false” (Morgan et al. 1991). Some say that these solutions answer a slightly different question – one phrasing is “you have to announce *before a door has been opened* whether you plan to switch” (Gillman 1992, emphasis in the original).

The simple solutions show in various ways that a contestant who is determined to switch will win the car with probability 2/3, and hence that switching is the winning strategy, if the player has to choose in advance between “always switching”, and “always staying”. However, the probability of winning by *always* switching is a logically distinct concept from the probability of winning by switching *given the player has picked door 1 and the host has opened door 3*. As one source says, “the distinction between [these questions] seems to confound many” (Morgan et al. 1991). This fact that these are different can be shown by varying the problem so that these two probabilities have different numeric values. For example, assume the contestant knows that Monty does not pick the second door randomly among all legal alternatives but instead, when given an opportunity to pick between two losing doors, Monty will open the one on the right. In this situation the following two questions have different answers:

- What is the probability of winning the car by
*always*switching? - What is the probability of winning the car
*given the player has picked door 1 and the host has opened door 3*?

The answer to the first question is 2/3, as is correctly shown by the “simple” solutions. But the answer to the second question is now different: the conditional probability the car is behind door 1 or door 2 given the host has opened door 3 (the door on the right) is 1/2. This is because Monty’s preference for rightmost doors means he opens door 3 if the car is behind door 1 (which it is originally with probability 1/3) or if the car is behind door 2 (also originally with probability 1/3). For this variation, the two questions yield different answers. However as long as the initial probability the car is behind each door is 1/3, it is never to the contestant’s disadvantage to switch, as the conditional probability of winning by switching is always at least 1/2. (Morgan et al. 1991)

Players who STAY have won 49040 cars out of 145987 games yielding a winning percentage of 34%

players who SWITCH have won 68356 cars out of 103063 games yielding a winning percentage of 66%

## Video Transcript:

You’re on a game show and there are three doors in front of you. The host, Monty Hall, says, “Behind one door is a brand new car. Behind the other two doors are goats. Pick a door!” You think, “Well, it doesn’t matter which door I choose, every door has a 1/3 chance of having the car behind it.” So, you choose door number 1. Now it gets interesting. Monty, the host, who knows where the car is, opens door number 2 and reveals a goat. The host always opens a door to reveal a goat. The host says, “If you want, you can switch to door number 3.” What should you do? Stay with your original choice or switch to the other door? All right, so what are you going to do? Stay or switch? Well, it’s a fifty-fifty chance of winning the car in either door. Right? [Wrong!] You actually double your chances of winning the car by switching doors. And that is why the Monty Hall Problem is so evasive!

Choose an explanation to the Monty Hall Problem:

1/3 vs 2/3 – Solution #1 to the Monty Hall Problem

There is a 1/3 chance of the car being behind door number 1 and a 2/3 chance that the car isn’t behind door number 1. After Monty Hall opens door number 2 to reveal a goat, there’s still a 1/3 chance that the car is behind door number 1 and a 2/3 chance that the car isn’t behind door number 1. A 2/3 chance that the car isn’t behind door number 1 is a 2/3 chance that the car is behind door number 3.

100 Doors! – Solution #2 to the Monty Hall Problem

Imagine that instead of 3 doors, there are 100. All of them have goats except one, which has the car. You choose a door, say, door number 23. At this point, Monty Hall opens all of the other doors except one and gives you the offer to switch to the other door. Would you switch? Now you may arrogantly think, “Well, maybe I actually picked the correct door on my first guess.” But what’s the probability that that happened? 1/100. There’s a 99% chance that the car isn’t behind the door that you picked. And if it’s not behind the door that you picked, it must be behind the last door that Monty left for you. In other words, Monty has helped you by leaving one door for you to switch to, that has a 99% chance of having the car behind it. So in this case, if you were to switch, you would have a 99% chance of winning the car.

Pick a Goat – Solution #3 to the Monty Hall Problem

To win using the stay strategy, you need to choose the car on your first pick because you’re planning to stay with your initial choice. The chance of picking the car on your first pick is clearly one out of three. But, in order to win using the switch strategy, you only need to pick a goat on your first pick because the host will reveal the other goat and you’ll end up switching to the car. So you want to use the strategy that lets you win if you choose a goat initially because you’re twice as likely to start by picking a goat.

Scenarios – Solution #4 to the Monty Hall Problem

To understand why it’s better to switch doors, let’s play out a few scenarios. Let’s see what will happen if you were to always stay with your original choice. We’ll play out three scenarios, one for each door that the car could be behind (door number 1, door number 2, or door number 3). And it doesn’t matter which door you start out with, so, to keep it simple, we’ll always start by choosing door number 1.

Stay strategy, scenario 1: the car is behind door number 1. You choose door number 1, then the host reveals a goat behind door number 2 and because you always stay, you stay with door number 1. You win the car! Stay strategy, scenario 2: the car is behind door number 2. You start by picking door number 1, the host reveals a goat behind door number 3, and you’re using the stay strategy so you stay with door number 1. You get a goat and don’t win the car. Stay strategy, scenario 3: the car is behind door number 3. You pick door number 1, the host opens door number 2 to reveal a goat, you stay with door number 1, and you get a goat. So, using the stay strategy, you won the car one out of three times. That means that in any one instance of playing the game, your chance of winning the car if you choose to stay is 1/3 or about 33%.

Now let’s try switching doors. Again, we’ll always start by picking door number 1. Switch strategy, scenario 1: the car is behind door number 1. You choose door number 1, the host opens door number 2 to reveal a goat, you are using the switch strategy so you switch to door number 3. You get a goat. Switch strategy, scenario 2: the car is behind door number 2. You start by picking door number 1, the host opens door number 3 to reveal a goat, you switch to door number 2 and win the car! Switch strategy, scenario 3: the car is behind door number 3. You pick door number 1, the host opens door number 2 to reveal a goat, you switch to door number 3 and win the car again! So, with the switch strategy you won the car 2 out of 3 times. That means, that in any one instance of the game, your chance of winning the car if you choose to switch doors is 2/3 or about 67%.

Therefore, if you play the game three times and stay, on average you’ll win the car once. But if you play the game three times and switch each time, on average you’ll win the car twice. That’s twice as many cars!