Three magnetic compasses are placed at the corners of a horizontal equilateral triangle. as in any ordinary compass, each compass
needle is a magnetic dipole constrained to rotate in a horizontal
plane. in this case the earth’s magnetic field has been precisely
annulled. the only field that acts on each dipole is that of the other
two dipoles. what orientation will they eventually assume? (use
symmetry arguments! ) can your answer be generalized for n compasses at the vertices of an n-gon?

An alpha particle with kinetic energy 12.5 mev makes a collision with lead nucleus, but it is not "aimed" at the center of the lead nucleus, and has an initial nonzero angular momentum (with respect to the stationary lead nucleus) of magnitude l = p0 b, where p0 is the magnitude of the initial momentum of the alpha particle and b=1.40×10−12 m. (assume that the lead nucleus remains stationary and that it may be treated as a point charge. the atomic number of lead is 82. the alpha particle is a helium nucleus, with atomic number 2.) what is the distance of closest approach?