问题 选择题

下列描述的物体中,受到平衡力作用的物体是(  )

A.正在加速向上飞的运载火箭

B.正在减速的汽车

C.正在沿弧形轨道快跑的四驱车

D.静止在桌面上的粉笔盒

答案

A、因为正在加速向上飞的运载火箭,因此火箭受到非平衡力的作用;

B、因为汽车正在做减速运动,因此汽车受非平衡力作用;

C、因为四驱车沿弧形轨道快跑,运动方向一直在改变,因此受非平衡力作用;

D、因为粉笔盒静止在桌面上,因此粉笔盒受平衡力作用.

故选 D.

判断题
单项选择题

When Dave was eighteen, he bought a secondhand car for 200 so that he could travel to and from work more (1) than by bus. It worked quite well for a few years, but then it got so old, and it was costing him (2) much in repairs that he decided that he had better (3) it.

He asked among his friends to see if anyone was particularly (4) to buy a cheap car, but they all knew that it was falling to pieces, so (5) of them had any desire to buy it. Dave’s friend Sam saw that he was (6) when they met one evening, and said, "What’s (7) , Dave"

Dave told him, and Sam answered, "Well, what about advertising it in the paper You may (8) more for it that way than the cost of the advertisement!" Thinking that Sam’s (9) was sensible, he put an advertisement in an evening paper, which read "For sale: small car, (10) very little petrol, only two owners. Bargain at 50. " For two days after the advertisement first appeared, there was no (11) . But then on Saturday evening he had an enquiry. A man rang up and said he would like to (12) him about the car. "All right, " Dave said, feeling happy. He asked the man whether ten o’clock the next morning would be (13) or not. "Fine, " the man said, "and I’ll (14) my wife. We intend to go for a ride in it to (15) it. "

The next morning, at a quarter to ten, Dave parked the car in the square outside his front door, (16) to wait there for the people who had (17) his advertisement. Even Dave had to (18) that the car really looked like a wreck. Then, soon after he had got the car as clean (19) it could be, a police car stopped just behind him and a policeman got out. He looked at Dave’s car and then said, "Have you reported this (20) to us yet, sir\

12()

A.tell 

B.see 

C.agree 

D.call