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By Hamish Johnston and Michael Banks
This week’s Red Folder opens with a fantastic video (above) from the folks at Veritasium. It involves dropping a spinning basketball from the top of a very tall dam in Tasmania and watching as the ball accelerates away from the face of the dam before bouncing across the surface of the water below. In comparison, a non-spinning ball simply falls straight down. This happens because of the Magnus effect, which has also been used to create flying machines and sail-free wind-powered boats. The effect also plays an important role in ball sports such as tennis and is explained in much more detail in our article “The physics of football”.
The idea of scientists destroying the world by accident is a popular theme in science fiction – and it can sometimes spill out into the real world. Consider the case of the US lawyer who tried to stop the Large Hadron Collider from starting up in case it produced a black hole that swallowed up the Earth (see “Law and the end of the world”). Another persistent apocalypse myth is that the physicists who detonated the first nuclear bomb in July 1945 thought it was possible that the explosion could set off a nuclear chain reaction that would consume the Earth’s atmosphere. Indeed, Enrico Fermi was apparently taking bets at the Trinity site in New Mexico about whether the bomb would destroy the entire planet or just that state of New Mexico. It turns out that Manhattan Project physicists had already worked out that an atmosphere-destroying chain reaction was impossible. However, that didn’t stop James Courant – head of the Manhattan Project – from thinking he had destroyed the world just after the bomb went off 70 year ago. See “The man who feared, rationally, that he’d just destroyed the world” in the Washington Post by Joel Achenbach.
But what if Courant and company did manage to destroy the atmosphere; is there a nearby planet that humanity could flee to? Yesterday, NASA’s Kepler mission announced the discovery of a near-Earth-size planet in the habitable zone of a Sun-like star that could fit the bill. Called Kepler-452b, it orbits its star in 385 Earth days and has a diameter about 60% larger than that of Earth. Most importantly, it is in its star’s habitable zone, so in principle it could have the right atmosphere to harbour life. This week we released a podcast with exoplanet expert Sara Seager of the Massachusetts Institute of Technology in which she explains how astronomers will look for signs of life on Kepler-452b and other distant worlds (“Searching for life on other planets”).
Earth 2.0: an artist’s impression of Kepler- 452b (right) compared with Earth. (Courtesy: NASA/JPL-Caltech/T Pyle)
Is your love of astrophysics strong enough to make you spend a damp Manchester winter living in a tent? That’s what Canadian student Evan Eames did when he couldn’t afford housing costs on top of international tuition fees of almost £20,000 during his Master’s degree at the University of Manchester. After appealing for help on Gumtree, for 10 months he pitched his tent in the backyard of Charley Mantack. In exchange, 24-year-old Eames tutored Mantack in maths and physics for a GCSE qualification she was studying for at college. Apart from spending the odd night on Mantack’s sofa, Eames would use the shower in the university’s Alan Turing building to keep clean, telling The Manchester Evening News that he found the whole experience “enjoyable”.
Eames has answered questions about his stay on reddit and one of the most popular queries was whether his tent left a dead patch on Mantack’s grass. Eames is now moving to Paris to do a PhD in physics. Let’s hope that he finds more suitable accommodation.
Finally, if a damp January night in Manchester isn’t chilly enough for you, how about a cryotherapy session at a frigid –160 °C. That’s what a mostly naked Toronto Star reporter Michael Robinson did this week in the name of journalism. He was investigating the latest celebrity-endorsed craze whereby the skin is exposed for two minutes to air that is cold enough to chill some types of superconductor. What really made me smile is that he paraphrased that old Canadian cliché “it’s a dry cold” to explain why his skin didn’t freeze instantly. If you want to try this yourself, please heed Robinson’s warning: “Currently, cryotherapy is not a licensed profession nor is it regulated by Health Canada.”
The Magnus effect comes straight from the Bernoulli’s law in the domain of fluid dynamics: it give the up-lift to the aeroplanes, when the air flow below and above their wings creates an upwards pressure differenc; due to this effect the spinning football or tennis ball goes off its straight line; it may even help the birds in their flight.
If that were true, how can planes fly upside down? I know. It’s the conventional textbook explanation, but it’s WRONG as can be.
The same reason that an upright plane can fly down. When you tilt the wing against the airstream it creates a vacuum on the underside when tilting down or on the upperside when tilting up, to suck the plane down or up respectively.
I think the main thing behind aerofoil lift is simple deflection; the assymetric shape of an aerofoil gives the wing a “twist” that pilots find useful, especially when taking off, as it increases the angle of attack, bringing the aerofoil into a greater lift domain. This extra twist is not big enough to stop aircraft flying upside down.
This explanation, along with that of MJBridger makes sense in a two dimensional approximation, and may give a partial answer. Aeronautical engineers, however, deal with three dimensional wings, which produce more complex and interesting physics.
https://en.wikipedia.org/wiki/Lifting-line_theory
https://en.wikipedia.org/wiki/Horseshoe_vortex
This has led to the design of more efficient wings.
Not quite ‘straight from the Bernoulli’s law’. To produce the Magnus effect (or any lift or form drag effect), there has to be boundary layer separation, which can only happen if the boundary layer exists – and the boundary layer is, by definition, the region where viscosity is significant and Bernoulli’s equation doesn’t apply.
(see Faber, 1995, _Fluid dynamics for physicists_, Cambridge University Press, Cambridge, section 7.12)
I don’t see how the Bernouilli or Magnus effect can significantly contribute to aerofoil lift, despite what textbooks say. For the former, there is almost no component of the forward force in the direction normal to the wing surface needed to produce lift. For the latter, the wing is not spinning, so I don’t see how the Magnus effect applies. Golfballs yes, but not an aerofoil. Could then lift from an an aircraft wing not be as I stated above, mostly simple deflection?
Kepler 425b sounds very interesting. Being 1.5 billion years older than Earth and with 2.5 times the surface area and possibly twice the surface gravity it might support much more life and more advanced and powerful life than Earth. There could have been powerful and advanced civilisations there over a billion years ago. So conceivably they could have sent something from there to Earth long ago. It only takes 28 million years with our technology but with theirs maybe a lot less. The planet is in the Cygnus constellation near the beak of the swan. I recall browsing a book specifically about Cygnus as the constellation held by many ancient cultures to be especially important.
how does the magnus effect affect long distance target shooting? the projectile wouldn’t just drift left and right would it? if the projectile were spinning clockwise and the wind were blowing from right to left it would lift the projectile?