Jeux / Puzzles

Journal La Recherche, juin 1999 - octobre 2003, en alternance avec Thierry Charitât et Pierre Sens

Shampoing sur table

La Recherche, 358, 2002, p.103 / Solution, 359, 2002, p.
If you put some shampoo on your table and squeeze it with the lid of a CD, you can watch some nice air fingers while lifting the lid (Saffman-Taylor type of instability). When you empty your shampoo bottle and get only air out, is it the same phenomenon ?

Le chêne et le Soleil

La Recherche, 357, 2002, p.79 / Solution, 358, 2002, p.103
Plants compete to get more light. In a tree, leaves at the top may hinder photosynthesis in leaves further down, yet it is in the interest of the tree that all leaves get enough light. Oak leaves are more indented at the top than lower in the tree : is that related ?

Chipolata de baudruche

La Recherche, 355, 2002, p.108 / Solution, 356, 2002, p.79
Cylindrical balloons usually display two stable radii. When they coexist, it is analogous to a first order phase transition. Let us compare this with water and ice : what is the analog of fusion temperature, latent heat, etc ?

Le sable qui boycotte

La Recherche, 353, 2002, p.76 / Solution, 354, 2002, p.81
Avalanches in the mountain illustrate segregation in granular matter : big rocks roll down to the bottom, while smaller stones settle earlier along the slope. If you shake a mixture of grains, they will typically segregate. Now suppose you have a mixture of grains in a sealed test tube : there exists a way to mix them efficiently, it is the granular analog to the Boycott effect originally discovered in blood sedimentation.

Quand le papier est-il dur ?

La Recherche, 351, 2002, p.76 / Solution, 352, 2002, p.
A bus ticket is flexible, but you can fold it into something very rigid. Why is that so ? What about cardboard ? What about ping-pong balls ?

Un dîner très compromis...

La Recherche, 349, 2002, p.76 / Solution, 351, 2002, p.76
A stupid adhesive label on your new saucepan may well delay your meal. Why is it so hard to detach ? Why is there always something left ?

N'en mettez pas une tartine...

La Recherche, 347, 2002, p.76 / Solution, 349, 2002, p.77
Do you like honey ? You probably often get drops running down your toast ! Why do drops form ? Why is your wall usually exempt of solidified paint drops ?

Avec quoi peut-on jongler ?

La Recherche, 344, 2002, p.110 / Solution, 345, 2002, p.75
When you throw an object up in the air, the way it rotates depends on its momentum of inertia. Is it easier to juggle with rods, with rings or with plates ?

Quand l'eau arrête les ondes

La Recherche, 342, 2001, p.74 / Solution, 343, 2001, p.
When the water jet from the tap encounters your finger, you can see that it hosts some stationary waves. What happens if you put liquid soap on your finger ?

Des rameurs sachant ramer

La Recherche, 340, 2000, p.89 / Solution, 341, 2001, p.91
Is a boat more rapid with a single rower or with eight ? McMahon and Bonner (On Size and Life, 1983) show that for boats of similar shapes, the velocity should scale like the 1/9th power of the number of rowers. They collected data from top-level races. They fit very nicely on this power-law. Why is the scaling theory sucessful, even though the number of rowers does not even span one decade ? Because the boats are optimized and are therefore of highly similar shapes, and because the highly trained rowers behave in a similar manner. If ants operate a boat on a sea of honey, how does the velocity depend on the number of ants ? How far does the new law extend ?

Quand l'eau s'incurve...

La Recherche, 337, 2000, p.94 / Solution, 338, 2001, p.95
Why does a sponge remove water from the table ? Inside its pores, the water free surface has a large curvature, and surface tension then induces a low pressure which pulls the water up. Conversely, if you put a paper-clip delicately on the water, the free surface bends downwards and the clip floats ! The shape of the water free surface is then described by the same equation as the motion of a pendulum or the form of a long, flexible body subjected to a force. In this way also does the water-strider float. Is the velvet on its legs of any help ?

Un câble bien accroché

La Recherche, 335, 2000, p.94 / Solution, 336, 2000, p.94
You must use strong hooks to hang a wire from the ceiling. Strong in what sense ? They must resist force (the weight of the wire) but also energy, so that the fracture does not propagate whenever one hook does break. Suppose you want to use a more rigid wire : should the hook threshold force or energy be increased ?

Peut-on faire flamber un jet d'eau ?

La Recherche, 334, 2000, p.94 / Solution, 335, 2000, p.95
Because it is elastic and thin, a rod buckles when subjected to a strong enough compressive force along its axis : it prefers to bend rather than to be further compressed. The same happens for an rope that whirls on the ground. When honey whirls onto a slice of bread, is it similar ? Yes : researchers recently showed that such a viscous fluid can also transmit torques and buckle. But a water jet never whirls : why ?

Au bord de la rivière

La Recherche, 329, 2000, p.96 / Solution, 330, 2000, p.95
Stones fall through water : a liquid does not resist shear. Yet they do not fall through sand : sand resists shear due to friction between the non-deformable grains. But since water molecules are also essentially non-deformable and very close to each other, why is water actually so fluid ?

Ca grince ou ca chante ?

La Recherche, 327, 2000, p.96 / Solution, 328, 2000, p.125
Common noises often originate in a stick-slip phenomenon. Can you easily tell whether the period of “stick” or the period of “slip” is longer ?

Le pèse-lettres qui joue au pendule

La Recherche, 325, 1999, p.99 / Solution, 326, 1999, p.98
Take a strip of paper and attach a letter to its end. Fix the other end under a heavy book on top of a shelf. The form of the strip is then analogous to the motion of a simple pendulum ! What is the equivalent of small oscillations and of continuous rotation ?

Peut-on avoir raté l'éclipse ?

La Recherche, 323, 1999, p.98 / Solution, 324, 1999, p.101
Inside the strip of land where the eclipse was total, the luminosity seemed to decrease very abruptly when the Sun was progressively hidden. Yet in regions where the eclipse was only partial, its variations appeared very gentle : you almost could have missed it ! Why is that so different ?

Chaud ou froid, l'oeuf à la coque ?

La Recherche, 321, 1999, p.102 / Solution, 322, 1999, p.123
Can you prepare a boiled egg in such a way that once you have it in your hand, you feel it alternatively warm and cool several times ?

diffusion/jeux_de_physique.txt · Dernière modification: 2009/01/09 20:29 par cgay
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