Earth’s uneven layers
The rotation of Earth’s solid inner core may have recently paused relative to the mantle and crust and now appears to be reversing direction, Nikk Ogasa reported in “Earth’s inner core may ‘reverse’ its spin” (SN: 2/25/23, p. 7).
The inner core’s rotation is thought to be partly influenced by the mantle’s gravitational pull, Ogasa reported. Reader Robert G. Chester wondered how that could be true given the shell theorem, which states that a body inside a spherically symmetrical shell should experience a net-zero gravitational force.
The mantle indeed serves as a shell around the core. And if Earth’s layers were perfectly spherical and symmetrical, the inner core would not experience a gravitational force as it rotates, says geophysicist Yi Yang of Peking University in Beijing. But Earth’s structure is not uniform. The mantle is elliptical and its density varies, so the gravitational forces acting on the inner core at different points are not exactly the same, Yang says.
Plant power
Thanks to special motorlike cells, Mimosa pudica plants can close their feathery leaflets like a book when touched, Susan Milius reported in “How ‘plant muscles’ fold up a mimosa” (SN: 2/25/23, p. 13).
Reader Van Snyder mused over the variety of ways that plants move, from leaves that follow the sun to the killer close of Venus flytraps. Do these plant motions use different mechanisms?
Various mechanisms can drive plant motion, says biomechanist David Sleboda of the University of California, Irvine. “Sunflowers, for example, track the movement of the sun by only growing one side of their stem at a time, causing the stem to bend back and forth over the course of a day,” Sleboda says. In contrast, Venus flytraps use pressure building in specialized cells. The trap’s bowed shape increases speed and power for a “snap-buckle” close when prey enters the trap, he says.
While these…
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