OPAC - Artikel Jurnal & Majalah

Pengarang	:	-
Nama Majalah/Jurnal	:	American Journal of Physics
Volume / Edisi	:	89 (No. 10)
Halaman	:	927–929
Abstrak	:	A novel graphical depiction of the relativistic barn and pole paradox is constructed in such a way that the space-time plot axes are dimensionless for both observers. Times of events over the entire range of pole-to-barn length ratio and relative speeds can be displayed in one plot for each observer. That for the barn frame clearly depicts the range of pole-to-barn length ratio over which the paradox appears, while that for the pole frame shows that the pole vaulter will only see the pole within the barn if the proper length of the pole is smaller than the contracted length of the barn.

Pengarang	:	Andrei A. Snarskii
Nama Majalah/Jurnal	:	American Journal of Physics
Volume / Edisi	:	89 (No. 10)
Halaman	:	916-920
Abstrak	:	Conventional calculations of the inertia tensor in undergraduate physics course are usually done for highly symmetrical bodies. Students might therefore get the impression that the moment of inertia about any axis through the center of mass is the same only for bodies with the highest degree of symmetry relative to this point, e.g., for spheres. A simple, seemingly counterintuitive example is presented, showing that the moment of inertia of a non-regular body, here an assembly of material points, can be the same about any axis passing through its center of mass.

Pengarang	:	-
Nama Majalah/Jurnal	:	American Journal of Physics
Volume / Edisi	:	89 (No. 10)
Halaman	:	909-915
Abstrak	:	Within a vehicle in free fall, such as a spacecraft orbiting the Earth, everything appears as if there was no force of gravity: objects float and remain stationary with respect to the walls, a situation commonly described as “weightlessness.” In relativity, such a vehicle is considered an inertial reference system. In fact, this is not exactly true because of the non-uniformity of the gravitational field. Tidal effects and forces appear, and complicated motions are generated. This article examines these effects, limiting the use of complicated mathematical formalism in favor of physical reasoning. Two particular cases are treated: a spacecraft that rotates about its center of mass in sync with its orbit, so it always faces the Earth the same way, and a spacecraft that maintains the same orientation with respect to the fixed stars throughout its orbit. The static problem is solved by finding the tidal forces acting on an object at different positions within the spacecraft. The trajectories of objects released from various positions within the spacecraft are then calculated. Some concluding conceptual observations are given.