问题 计算题

汽车刹车后,停止转动的轮胎在地面上滑动,其滑动的痕迹可以明显地看出,这就是我们常说的刹车线.由刹车线的长短可以得知汽车刹车前后速度大小,因此刹车线的长度是分析交通事故的一个重要依据.某汽车质量为1.0×103 kg,刹车前正在做匀速直线运动,运动中所受阻力是车重力的0.1倍.若刹车后在滑动过程中该车所受阻力是车重力的0.7倍,刹车线长14 m,g取10 m/s2,求:

(1)刹车前该汽车的速度大小;

(2)刹车前该汽车牵引力的功率.

答案

(1)14 m/s  (2)1.4×104 W

(1)刹车后,由动能定理有f·l=

解得

(2)刹车前,牵引力F=f=0.1mg

由P=Fv

解得P=1.4×104 W.

问答题
阅读理解

     People are overweight for many reasons, the most important one of which is that they eat the wrong

foods, get the wrong types of calories per meal, and they also eat meals at the wrong time each day.

Food is more powerful than any weight loss pill, because the food that you eat can either make you thin

or fat. This is true because your body is like an engine, and it only needs certain foods at certain intervals(间隔)each day.

     Low-calorie Diets Do Not Work

The reason people cannot lose weight by starving themselves is that their metabolism(新陈代谢)will

detect any major drop in calories and it will then adjust(调整)itself by burning fewer calories each day.

That is the reason why you can eat 1,000 calories per day and not lose any weight, while your friends

can eat 2,500 calories per day and not gain any weight.

     Low-fat Diets Do Not Work

     Many people in today's society are buying mostly low- fat or non-fat food at the grocery store.

Everybody knows how much fat is inside the food they buy. However, people are getting fatter than ever

before by doing this. They are not losing weight by changing to this low-fat lifestyle.                  

     Losing weight has nothing to do with these things. One of the most important things you can do to

control your weight is to eat the proper foods, such as fruit and vegetables, eat them at the right time, and

exercise every day. If people make this a habit, they will lose weight.

1. What is the main reason why people are overweight according to the author?

A. They eat the wrong foods at the wrong time.

B. They eat food with lots of calories.

C. They don't take any weight loss pills.

D. They burn many calories every day.

2. The article is written in order to _______.

A. tell readers not to buy low-fat or non-fat food

B. tell readers how to lose weight

C. tell readers about an important scientific discovery

D. call on people to lose weight and stay healthy

3. To realize the aim of your weight loss plan, you should ________.

a. do exercise every day      

b. eat meals at the right time

c. eat fruits and vegetables    

d. eat low-fat or sweet food

e. eat the proper foods at each meal

A. a, b, c, d              

B. a, b, d, e    

C. a, c, d, e              

D. a, b, c, e