两颗靠得较近的天体组成双星，它们以两者连线上某点为圆心做匀速圆周运动，因而不会由于相互的引力作用而被吸到一起，忽略其它天体对该双星系统的影响．则下面说法正确的是（ ）
A．它们做圆周运动的角速度之比与它们的质量成反比
B．它们做圆周运动的线速度之比与它们的质量成反比
C．它们做圆周运动所需的向心力之比与它们的质量成反比
D．它们做圆周运动的半径之比与它们的质量成正比
A、因为双星各自做匀速圆周运动的周期相同，根据角速度与周期的关系可知双星的角速度之比为1：1，故A错误
B、双星做匀速圆周运动的向心力由万有引力提供，
向心力F=mω2r，双星的向心力之比为1：1，
∴双星的轨道半径之比等于它们的质量成反比，
根据v=ωr得它们做圆周运动的线速度之比与它们的质量成反比，故B正确，C、D错误
故选B．
| The field of medicine has always attracted its share of quacks and charlatans — disrepu-table women and men with little or no medical knowledge who promise quick cures at cheap prices. The reasons why quackery thrives even in modem times are easy to find. To begin with, pain seems to be a chronic human condition. A person whose body or mind "hurts" will often pay any amount of money for the promise of relief. Second, even the best medical treatment cannot cure all the ills that beset men and women. People who mistrust or dislike the truths that their physicians tell them often turn to more sympathetic ears. Many people lack the training necessary to evaluate medical claims. Given the choice between (a) a reputable physician who says a cure for cancer will be long, expensive and may not work at all, and (b) a salesperson who says that several bottles of a secret formula "snake oil" will cure not only cancer but tuberculosis as well, some individuals will opt for "snake oil". Many "snake oil" remedies are highly laced with alcohol or narcotic drugs. Anyone who drinks them may get so drunk or stoned that they drown their pains in the rising tide of pleasant intoxication. Little wonder that "snake oil" is a popular cure-all for minor aches and hurts! But let there be no misunderstandings. A very few "home remedies" actually work. However, most remedies sold by quacks are not only useless, but often can be harmful as well. |
According to the author, a very few home remedies are______.
A．useless
B．harmful
C．pleasant
D．effective