问题 选择题

小丽推着购物车在超市购物,当她用20牛的水平推力使购物车以1.5米/秒速度沿直线匀速前进时,突遇情况想尽快站住,于是她用8牛的水平拉力使购物车慢慢停了下来,则在这一过程中,下列说法正确的是(  )

A.小车受到的摩擦阻力为8N

B.小车受到的摩擦阻力为12N

C.小车受到的合力大小为12N

D.小车受到的合力大小为28N

答案

A、B、小丽用20牛的水平推力使购物车以1.5米/秒速度沿直线匀速前进时,购物车水平方向上受到推力和摩擦力阻力作用,这两个力是平衡力,大小相等,所以摩擦力阻力是20N.A和B两个选项都是错误的.

C、D、遇到紧急情况想尽快站住,于是小丽用8牛的水平拉力使购物车慢慢停了下来,购物车受到水平向后的拉力和摩擦力作用,压力不变,接触面粗糙程度不变,摩擦力保持20N不变,所以购物车受到的合力为:F=20N+8N=28N,方向与购物车运动方向相反.故选项C错误,选项D正确.

故选D.

填空题

For years pediatricians didn’t worry much about treating hypertension in their patients. After all, kids grow so fast, it’s hard keeping up with their shoes size, let alone their blood pressure. Sure, hypertension in adults places them at greater risk of heart attack and stroke. But nobody likes the idea of starting youngsters on blood-pressure medicine they could wind up taking the rest of their lives. Who knows what previously unheard-of side effects could crop up after five or six decades of daily use

The rationale has been: kids grow out of so many things; maybe they’ll grow out of this too.

41.Now, though, comes word that high blood pressure can be destructive even in childhood.

42.Who is most at risk

Boys are more than girls, especially boys who are overweight. Their heart works so hard to force blood through extra layers of fat that its walls grow denser. Then, after decades of straining, it grows too big to pump blood very well.

43.How can you tell if yours are like the 670,000 American children ages 10 to 18 with high blood pressure

It’s not the sort of thing you can catch by putting your child’s arm in a cuff at the free monitoring station in your local grocery. You should have a test done by a doctor, who will consult special tables that indicate the normal range of blood pressure for a particular child’s age, height and sex.

44.About half the cases of hypertension stem directly from kids being overweight.

45.How can you do

You can keep your children from joining their ranks by clearing the junk food from your pantry and hooking you kids — the earlier the better — on healthy, attractive snakes like fruits (try freezing some grapes/or carrot sticks with salsa. ) Not only will they lower your children’s blood pressure: These foods will also boost their immune system and unclog their plumbing.

[A] And the problem is likely to grow. Over the past 30 years the proportion of children in the U. S. who are overweight has doubled, from 5% to 11% or 4.7 million kids.

[B] According to a recent report in the journal Circulation, 19 of 30 children with high blood pressure developed a dangerous thickening of the heart muscle that, in adults at least, has been linked to heart failure. “No one knows if this pattern holds true for younger patients as well, ” says Dr. Stephen Daniels, a pediatric cardiologist who led the study at Children’s Hospital Medical Center in Cincinnati, Ohio. “But it’s worrisome. ”

[C] Feed your children nutritious foods three times a day to keep his immune system healthy. Make sure the meals include all the food groups to ensure they are getting plenty of vitamins.

[D] Fortunately the abnormal thickening can be spotted by ultrasound. And in most case, getting that blood pressure under control — through weight loss and exercise or, as a last resort, drug treatment — allows the overworked muscle to shrink to normal size.

[E] If the doctor finds an abnormal result he will repeat the test over a period of months to make sure the reading isn’t a fake. He’ll also check, whether other conditions, like kidney disease, could be the source of the trouble, because hypertension is hard to be detected. The National Heart, Lung and Blood Institute recommends annual blood-pressure checks for every child over age 3.

[F] High blood pressure isn’t just an issue for adults; keeping kids blood pressure levels in a healthy range is also important. In a new study reported in Hypertension, researchers found that increased exercise duration among kids leads to lower blood pressure levels, however the same cannot be said for increased intensity.

[G] Meanwhile, make sure your kids spend more time on the playground than with their PlayStation. Even if they don’t shed a pound, vigorous exercise will help keep their blood vessels nice and wide, lowering their blood pressure. And of course, they’ll be more likely to eat light and, exercise if you set a good example.

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问答题 简答题