问题 问答题

如图所示,由光滑细管组成的轨道固定在竖直平面内,其中AB段和BC段是半径为R的四分之一圆弧.一小球从距离水平地面高为H的管口D处静止释放,最后能够从A点水平抛出落到地面上.若R可以变化,求:

(1)R的最大值;

(2)小球从A点平抛落地时的水平位移x的最大值.

答案

(1)为使小球能够从A点平抛,需满足:mg+N=

m
v2A
R

且N≥0.

小球从D点静止释放运动到A点的过程中机械能守恒,故有:

mgH=mg•2R+

1
2
m
v2A

联立解得:R≤

2
5
H,所以R的最大值为
2
5
H

(2)小球从A点平抛,根据平抛运动的规律有:

水平方向上:x=vAt

竖直方向上:2R=

1
2
gt2

联立解得:x=

8R(H-2R)

显然,当R=

H
4
时,小球从A点平抛的水平位移有最大值 xmax=H.

答:(1)R的最大值是

2
5
H;

(2)小球从A点平抛落地时的水平位移x的最大值是H.

问答题 简答题
单项选择题

Euthanasia has been a topic of controversy in Europe since at least 1936. On an average of six times a day, a doctor in Holland practices "active" euthanasia: (1) administering a lethal drug to a (2) ill patient who has asked to be relieved (3) suffering. Twenty times a day, life prolonging treatment is withheld or withdrawn (4) there is no hope that it can (5) an ultimate cure. "Active" euthanasia remains a crime on the Dutch statute books, punishable (6) 12 years in prison. But a series of court cases over the past 15 years has made it clear that a competent physician who (7) it out will not be prosecuted.

Euthanasia, often called "mercy killing", is a crime everywhere in Western Europe. (8) more and more doctors and nurses in Britain, Germany, Holland and elsewhere readily (9) to practicing it, most often in the "passive" form of withholding or withdrawing (10) The long simmering euthanasia issue has lately (11) into a sometimes fierce public debate, (12) both sides claiming the mantle of ultimate righteousness. Those (13) to the practice see themselves (14) sacred principles of respect for life, (15) those in favor raise the banner of humane treatment. After years (16) the defensive, the advocates now seem to be (17) ground. Recent polls in Britain show that 72 percent of British (18) favor euthanasia in some circumstances. An astonishing 76 percent of (19) to a poll taken late last year in France said they would like the law changed to (20) mercy killings. Obviously, pressure groups favoring euthanasia and "assisted suicide" have grown steadily in Europe over the years.

19()

A.officials

B.citizens

C.respondents

D.interviewers