add ch 4 hw, vacuum instability with respect to tachyon pairs
update broken URL for Eot-Wash pendulum
add ch 5 hw nontrivial-curvature-in-one-plus-one
add ch 6 hw black-hole-slingshot
two different arguments for v=c for massless particles
revise treatment of relativistic momentum in ch 4
non-additivity of mass; revise expansion of bound systems
fix misplaced indices on stress-energy tensor; add discussion of signs and indices
rope in Schwarzschild example
ch 4, add relativistic work-KE theorem
fix errors in dangling rope example
fix mistake in rope example; add Maxima code