题干

在水平地面上运动的小车车厢底部有一质量为m1=1kg的木块,木块和车厢通过一根轻质弹簧相连接,并处于静止状态,弹簧被拉伸且弹力大小为5N。此时在车厢的顶部用一根细线悬挂一质量为m2的小球,现对小车施加一水平力,使小车由静止开始运动,发现细线与竖直方向的夹角θ由0逐渐增加到37°(sin37°=0.6,cos37°=0.8),则在这段时间内(  )

A:木块与小车始终保持相对静止,弹簧对物体的作用力始终没有发生变化

B:随着小车加速度的增加,木块所受摩擦力逐渐增大

C:当木块摩擦力为0时,tanθ=0.75

D:小车以7.5m/s2的加速度向左做匀加速直线运动时,物体受到的摩擦力为3.5N

上一题 下一题 0.0难度 选择题 更新时间:2010-11-29 02:10:55

答案(点此获取答案解析)

A

同类题5

阅读理解

    Why do Americans struggle with watching their weight, while the French, who eat rich food, continue to stay thin? Now a research by Cornell University suggests how life style and decisions about eating may affect weight. Researchers concluded that the French tend to stop eating when they feel full. However, Americans tend to stop when their plate is empty or their favorite TV show is over.

    According to Dr. Joseph Mercola, a health expert, the French see eating as an important part of their life style. They enjoy food and therefore spend a fairly long time at the table, while Americans see eating as something to be squeezed (挤进) between the other daily activities. Mercola believes Americans lose the ability to sense when they are actually full. So they keep eating long after the French would have stopped. In addition, he points out that Americans drive to huge supermarkets to buy canned and frozen foods for the week. The French, instead, tend to shop daily, walking to small shops and farmers' markets where they have a choice of fresh fruits, vegetables, and eggs as well as high-quality meats for each meal.

    After a visit to the United States, Mireille Guiliano, author of French Women Don't Get Fat, decided to write about the importance of knowing when to stop rather than suggesting how to avoid food. Today she continues to stay slim and rarely goes to the gym.

    In spite of (尽管) all these differences, evidence shows that recent life style changes may be affecting French eating habits. Today the rate of obesity (肥胖)—or extreme overweight — among adults is only 6%. However, as American fast food gains acceptance and the young refuse older traditions, the obesity rate among French children has reached 17% — and is growing.