Posted by
timothy
on from the hard-boiled-science dept.
SuperAbe writes: "According to an article in this week's issue of the journal Nature, mathematicians Keith Moffatt of the University of Cambridge and Yutaka Shimomura of Keio University, Yokohama, Japan have discovered why hard-boiled eggs spun on their side stand on end spontaneously: friction."
Simple explanation for physics geeks
by
Viadd
·
· Score: 4, Informative
I didn't know there was any mystery about spinning eggs standing up. I haven't read the Nature paper, but here is a simple explanation (for those who have had second year physics).
A spinning body has both angular momentum and rotational energy. For a given amount of rotational energy, it has the lowest angular momentum when its mass is closest to the spin axis, and the largest angular momentum when its mass is farthest from the spin axis. These correspond to the spin axis aligned with the length of the egg, and across the egg, respectively.
When you start an egg spinning on its side on a tabletop, it is in its largest angular momentum state for that particular amount of rotational energy.
Because the egg is not a perfectly symmetric ellipsoid, and because of reality, the point of contact with the table at any given time is not perfectly aligned with the spin axis. This causes a force which gives a torque that reduces the angular momentum. If the egg also slipped across the surface, it would lose energy due to dynamic friction, but the energy loss is minimal if the friction makes the contact point roll along.
The egg now has reduced angular momentum but unchanged rotational energy, and to compensate it has to bring its mass closer to its axis, which it does by bringing its long axis closer to its spin axis, i.e. standing up. This also reduces the rotational energy of the egg by converting some of it into potential energy (the center of gravity of the egg is raised) which helps bring things into balance.
Slashdot Mirror
A spinning body has both angular momentum and rotational energy. For a given amount of rotational energy, it has the lowest angular momentum when its mass is closest to the spin axis, and the largest angular momentum when its mass is farthest from the spin axis. These correspond to the spin axis aligned with the length of the egg, and across the egg, respectively.
When you start an egg spinning on its side on a tabletop, it is in its largest angular momentum state for that particular amount of rotational energy.
Because the egg is not a perfectly symmetric ellipsoid, and because of reality, the point of contact with the table at any given time is not perfectly aligned with the spin axis. This causes a force which gives a torque that reduces the angular momentum. If the egg also slipped across the surface, it would lose energy due to dynamic friction, but the energy loss is minimal if the friction makes the contact point roll along.
The egg now has reduced angular momentum but unchanged rotational energy, and to compensate it has to bring its mass closer to its axis, which it does by bringing its long axis closer to its spin axis, i.e. standing up. This also reduces the rotational energy of the egg by converting some of it into potential energy (the center of gravity of the egg is raised) which helps bring things into balance.
The eventual result is an egg standing on end.