My Photo


  • Jen-Luc Piquant sez: "They like us! They really like us!"

    "Explains physics to the layperson and specialist alike with abundant historical and cultural references."
    -- Exploratorium ("10 Cool Sites")

    "... polished and humorous..."
    -- Physics World

    "Takes 1 part pop culture, 1 part science, and mixes vigorously with a shakerful of passion."
    -- Typepad (Featured Blog)

    "In this elegantly written blog, stories about science and technology come to life as effortlessly as everyday chatter about politics, celebrities, and vacations."
    -- Fast Company ("The Top 10 Websites You've Never Heard Of")
Blog powered by Typepad
Bookmark and Share

« REPOST: fashionably late to the dating game | Main | by the sword »


I apologize in advance for nitpicking, but the catenary is technically not a parabola, as is stated in the second paragraph. Galileo Galilei, who tried to solve the catenary problem and failed, also thought the catenary was a parabola, so you're in excellent company.

Another way to approach the catenary problem, the way it is usually taught to physics students, is to begin with the observation that the catenary's shape is one which minimizes the total gravitational potential energy of the chain, or, equivalently, minimizes the height of the center of gravity of the chain. Any shape other than that of the hyperbolic cosine (cosh), such as Galileo's parabola or, say, a vee shape, would have a higher potential energy or center of gravity, and thus would not be favored. ( For more info see or ) Incidentally, many physics problems become much simpler when reformulated in terms of energy (a scalar quantity) rather than in terms of forces, "pure tensions," or shear stresses (which are vectors or 2nd-rank tensors).

The hyperbolic cosine function is just the average of two exponential functions, an exponentially growing one [exp(x)] and an exponentially decaying one [exp(–x)]. In equation form, cosh(x) = [exp(x) + exp(–x)]/2 .

>"Legend has it that in ancient Rome, whenever an arch was constructed, the architect who designed it was forced to stand underneath as the wooden supports were removed as a means of quality control. It was a terrific motivational tool: design it right, or the arch falls and crushes you."

Did the architect who designed the Gateway Arch have to undergo (I use that verb advisedly) a similar ordeal? If so, is this how the expression "to be under the cosh" meaning "to be under pressure [to succeed]" came about? No, but it might make for a good apocryphal anecdote for your next cocktail party.

By the way, no one goes to the APS March Meeting anymore. It's too crowded.

For a somewhat different example of the intersection between physics and architecture, you might be interested in this brief snippet from a lecture by Richard Feynman:

Thanks for both these helpful comments... now I know what a cosh is, and I plan to spread the unofficial "legend" of the architect of the Gateway Arch being forced to stand under it while being constructed. With any luck, it will show up on the Urban Legends site one day... :)

You're welcome. Actually, the story I was hoping to have you spread was the bit about the origin of the expression "under the cosh". But perhaps that would be too much of a betrayal of your English-major roots.

(By the way, that equation for the gateway arch, "y = 68.8(cosh0.01-1)", seems to be missing the variable x in the right-hand side. Thought you'd want to know.)

Interesting post - Kenneth Swyers actually succeeded in his attempt to land on top of the arch. But the next step was to parachute off the top of the arch, and the wind caught his chute and he slid down one of the legs and died.

To be pedantic: exponential growth is still exponential growth, even if measured at discrete intervals. Your bank account that compounds yearly will, after n years, have (initial balance)*(1 + interest)^n dollars in it, which could also be written (initial balance)*(exp(n * log (1+interest)). Continuous compounding can be treated as changing the effective rate parameter; there are a bunch of formulas here to translate between equivalent interest rates at different compounding rates in this wikipedia article:

The comments to this entry are closed.

Twitter Updates

    follow me on Twitter

    Physics Cocktails

    • Heavy G
      The perfect pick-me-up when gravity gets you down.
      2 oz Tequila
      2 oz Triple sec
      2 oz Rose's sweetened lime juice
      7-Up or Sprite
      Mix tequila, triple sec and lime juice in a shaker and pour into a margarita glass. (Salted rim and ice are optional.) Top off with 7-Up/Sprite and let the weight of the world lift off your shoulders.
    • Listening to the Drums of Feynman
      The perfect nightcap after a long day struggling with QED equations.
      1 oz dark rum
      1/2 oz light rum
      1 oz Tia Maria
      2 oz light cream
      Crushed ice
      1/8 tsp ground nutmeg
      In a shaker half-filled with ice, combine the dark and light rum, Tia Maria, and cream. Shake well. Strain into an old fashioned glass almost filled with crushed ice. Dust with the nutmeg, and serve. Bongos optional.
    • Combustible Edison
      Electrify your friends with amazing pyrotechnics!
      2 oz brandy
      1 oz Campari
      1 oz fresh lemon juice
      Combine Campari and lemon juice in shaker filled with cracked ice. Shake and strain into chilled cocktail glass. Heat brandy in chafing dish, then ignite and pour into glass. Cocktail Go BOOM! Plus, Fire = Pretty!
    • Hiroshima Bomber
      Dr. Strangelove's drink of choice.
      3/4 Triple sec
      1/4 oz Bailey's Irish Cream
      2-3 drops Grenadine
      Fill shot glass 3/4 with Triple Sec. Layer Bailey's on top. Drop Grenadine in center of shot; it should billow up like a mushroom cloud. Remember to "duck and cover."
    • Mad Scientist
      Any mad scientist will tell you that flames make drinking more fun. What good is science if no one gets hurt?
      1 oz Midori melon liqueur
      1-1/2 oz sour mix
      1 splash soda water
      151 proof rum
      Mix melon liqueur, sour mix and soda water with ice in shaker. Shake and strain into martini glass. Top with rum and ignite. Try to take over the world.
    • Laser Beam
      Warning: may result in amplified stimulated emission.
      1 oz Southern Comfort
      1/2 oz Amaretto
      1/2 oz sloe gin
      1/2 oz vodka
      1/2 oz Triple sec
      7 oz orange juice
      Combine all liquor in a full glass of ice. Shake well. Garnish with orange and cherry. Serve to attractive target of choice.
    • Quantum Theory
      Guaranteed to collapse your wave function:
      3/4 oz Rum
      1/2 oz Strega
      1/4 oz Grand Marnier
      2 oz Pineapple juice
      Fill with Sweet and sour
      Pour rum, strega and Grand Marnier into a collins glass. Add pineapple and fill with sweet and sour. Sip until all the day's super-positioned states disappear.
    • The Black Hole
      So called because after one of these, you have already passed the event horizon of inebriation.
      1 oz. Kahlua
      1 oz. vodka
      .5 oz. Cointreau or Triple Sec
      .5 oz. dark rum
      .5 oz. Amaretto
      Pour into an old-fashioned glass over (scant) ice. Stir gently. Watch time slow.