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一架质量为m的飞机，以v0的速度沿水平方向匀速飞行，此时飞机发动机的输出功率刚好为P0/8  （P0为发动机的额定功率）．接着飞机开始作水平方向的匀加速直线飞行，当速度增加到8v0/5时，发动机的输出功率恰好达到P0．已知飞机在飞行中所受到的阻力与飞机飞行的速率二次方成正比，试求：

（1）飞机飞行所能达到的最大速度vm；

（2）匀加速运动所经历的时间t0；

（3）简要叙述若要提高飞机的飞行速度，在设计时应考虑哪些因素？

答案

（1）据题，飞机在飞行中受到的阻力与飞机飞行的速率二次方成正比成正比，则有 f=kv2，k是比例系数．

当飞机以v0的速度匀速飞行时，

P0
8
=F1v0=kv03①

当飞机以额定功率匀速飞行时，速度达到最大，则有 P0=kvm3②

由①②得，vm=2v0

（2）飞机匀加速运动的加速度为a=

F-f
m
=
P0
8
5
v0
-k(
8v0
5
)2
m
③

飞机作匀加速运动所经历的时间为 t=

8v0
5
-v0
a
④

由①③④联立得，t=

120mv02
61P0

（3）在设计时应考虑增加发动机功率，减小阻力等可以提高飞机的飞行速度．

答：（1）飞机飞行所能达到的最大速度为2v0；

（2）匀加速运动所经历的时间为

120mv02
61P0
；

（3）在设计时应考虑增加发动机功率，减小阻力等可以提高飞机的飞行速度

单项选择题

心电图示心室颤动后，应立即（）

A.静注利多卡因

B.静注异丙基肾上腺素

C.电除颤

D.静注阿托品

E.安装临时起搏器
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填空题

Stress is our response to threatening or (1) events. We may experience different levels of stress depending on the (2) , meaning the events or circumstances that cause us to feel stress. Stressors can be classified into (3) main categories: (4) events, personal stressors, and (5) stressors. Cataclysmic events are major events that cause stress (6) , immediately, for a great many people at once. Personal stressors are major life events that create stress, including (7) life events. Background stressors are (8) hassles, or minor irritations, or (9) background stressors. Repeated exposure to stressors has both psychological and (10) consequences. The long- term consequence is that body tissues such as the heart and blood vessels can begin to (11) . The (12) system functions less effectively, and (13) our ability to fight off illnesses.
The General Adaptation (14) explains the sequence of physiological reactions to stress. There are three phases to G. A. S. The first is the " alarm and (15) " phase: we respond with alarm, and take (16) to remove the stressor. The second phase is the (17) stage: we fight against the stressor or try to cope with the stressor. This can lead to the third phase. (18) . The exhaustion phase may be an (19) way of trying to avoid the stressors. The body may be telling us that we need to do whatever is (20) to remove the stressor.

Stress is our response to threatening or (1) events. We may experience different levels of stress depending on the (2) , meaning the events or circumstances that cause us to feel stress. Stressors can be classified into (3) main categories: (4) events, personal stressors, and (5) stressors. Cataclysmic events are major events that cause stress (6) , immediately, for a great many people at once. Personal stressors are major life events that create stress, including (7) life events. Background stressors are (8) hassles, or minor irritations, or (9) background stressors. Repeated exposure to stressors has both psychological and (10) consequences. The long- term consequence is that body tissues such as the heart and blood vessels can begin to (11) . The (12) system functions less effectively, and (13) our ability to fight off illnesses.
The General Adaptation (14) explains the sequence of physiological reactions to stress. There are three phases to G. A. S. The first is the " alarm and (15) " phase: we respond with alarm, and take (16) to remove the stressor. The second phase is the (17) stage: we fight against the stressor or try to cope with the stressor. This can lead to the third phase. (18) . The exhaustion phase may be an (19) way of trying to avoid the stressors. The body may be telling us that we need to do whatever is (20) to remove the stressor.
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