问题 计算题

直升机沿水平方向匀速飞往水源取水灭火,悬挂着m=500kg空箱的悬索与竖直方向的夹角θ1=45°,直升机取水后飞往火场,加速度沿水平方向,大小稳定在a=1.5m/s2时,悬索与竖直方向的夹角θ2=14°(如图所示),如果空气阻力大小不变,且忽略悬索的质量,试求水箱中水的质量M。(取重力加速度g=10m/s2,sin14°≈0.242,cos14°≈0.970)

答案

解:直升机取水前,水箱受力平衡T1sinθ1-f=0①

      T1cosθ1-mg=0②

      由①②得f=mgtanθ1

      直升机返回,由牛顿第二定律T2sinθ2-f=(m+M)a④

      T2cosθ2-(m+M)g=0⑤

      由④⑤得,水箱中水的质量M=4.5×103kg

单项选择题

C

When Mary Moore began her high school in 1951, her mother told her, "Be sure and take a typing course, so when this show business thing doesn’ t work out, you’ 11 have something to rely on. " Mary responded in typical teenage fashion. From that moment on, "the very last thing I ever thought about doing was taking a typing course," she recalls.

The show business thing worked out, of course. In her career, Mary won many awards. Only recently, when she began to write Growing Up Again, did she regret ignoring her mom, " I don’ t know how to use a computer," she admits.

Unlike her 1995 autobiography, after all, her second book is less about life as an award-winning actress and more about living with diabetes (糖尿病). All the money from the book is intended for the Juvenile Diabetes Research Foundation (JDRF), an organization she serves as international chairman. "I felt there was a need for a book like this," she says. "I didn’ t want to lecture, but I wanted other diabetics to know that things get better when we’ re self-controlled and do our part in managing the disease. "

But she hasn’ t always practiced what she teaches. In her book, she describes that awful day, almost 40 years ago, when she received two pieces of life-changing news. First, she had lost the baby she was carrying, and second, tests showed that she had diabetes. In a childlike act, she left the hospital and treated herself to a box of doughnuts (甜甜圈). Years would pass before she realized she had to grow up-again-and take control of her diabetes, not let it control her. Only then did she kick her three-pack-a-day cigarette habit, overcome her addiction to alcohol, and begin to follow a balanced diet.

Although her disease has affected her eyesight and forced her to the sidelines of the dance floor, she refuses to fall into self-pity. "Everybody on earth can ask, ’why me’ about something or other," she insists. "It doesn’t do any good. No one is immune (免疫的) to heartache, pain, and disappointments. Sometimes we can make things better by helping others. I’ve come to realize the importance of that as I’ ve grown up this second time. I want to speak out and be as helpful as I can be. \

We can know that before 1995 Mary().

A. had two books published

B. received many career awards

C. knew how to use a computer

D. supported the JDRF by writing

多项选择题