问题 问答题

额定功率为80kW的汽车,在某平直公路上行驶的最大速度为20m/s,汽车的质量m=2×103 kg.如果汽车从静止开始做匀加速直线运动,经5s后刚好达到额定功率,之后以额定功率沿直线行驶,若汽车运动过程中所受阻力不变,g=10m/s2,求:

(1)汽车所受的恒定阻力;

(2)3s末汽车的瞬时功率;

(3)汽车启动后前10s内汽车的平均功率.

答案

(1)当汽车匀速行驶时有:f=

P
vm

解得:f=4×103N      

(2)P=Fv=(f+ma)at

代入数据可得:a=2m/s2

当t=3s时,可解得:P=48kW    

(3)汽车启动后前5s,汽车做功:W1=FS=(f+ma)

1
2
at12

汽车启动后,5s末到10s末,汽车做功:W2=Pt2

汽车启动后前10s内汽车的平均功率:

.
P
=
W1+W2
t1+t2

解得:

.
P
=60kW

答:(1)汽车所受的恒定阻力为4×103N;

(2)3s末汽车的瞬时功率为48kW;

(3)汽车启动后前10s内汽车的平均功率60KW.

单项选择题
单项选择题

Personality is to a large extent inherent. A-type parents usually (1) A-type offspring. But the environment must also have a profound effect (2) if competition is important to the parents, it is (3) to become a major factor in the lives of their children.

One place where children (4) A characteristics is school, which is, (5) its very nature, a highly competitive institution. Too many schools (6) the "win at all costs" moral standard and measure their success by sporting achievements. The current (7) for making children compete against their classmates or against the (8) produces a two-layer system, in which competitive A types seem in some way better than their B type fellows. Being too (9) to win can have dangerous consequences: remember that Pheidippides, the first marathon runner, (10) dead seconds after saying. "Rejoice, we conquer!"

(11) the worst form of competition in schools is the disproportionate emphasis on examinations. There is, for example, a (12) school that allows pupils to concentrate on those things they do well. The (13) of competition by examination are somewhat questionable, but competition in the certain (14) of failure is positively harmful.

(15) , it is neither practical nor desirable that all A youngsters change into B’s. The world needs types, and schools have an important duty to try to (16) a child’s personality to his possible future employment. It is top management.

If the preoccupation of schools with academic work was (17) , more time might be spent teaching children surer values. Perhaps selection (18) the caring professions, especially medicine, could be made less by good grades in chemistry and more by such (19) as sensitivity and sympathy. It is surely a mistake to choose our doctors exclusively from A type stock. B’s are important and should be (20) .

4()

A. draw on

B. get over

C. drag on

D.soak up