问题 选择题

下面正确的说法是(  )

A.两个共点力的合力,一定比原来的每一个力都大

B.两个共点力的合力,一定比原来的每一个力都小

C.两个共点力的合力,可能比原来的每个力都大,也可能比原来的每个力都小

D.两个共点力的合力,一定小于该两个力的大小之和,一定大于该两个力的大小之差

答案

A、同一直线、反方向二力的合力大小等于二力之差,因此合力大小不一定大于其中任何一个力,不符合题意;

B、同一直线、同方向二力的合力大小等于二力之和,即合力一定大于其中任何一个力,不符合题意;

CD、由AB选项可得,两个共点力的合力,可能比原来的每个力都大,也可能比原来的每个力都小,故C选项说法正确,D选项说法错误;

故选C.

阅读理解

B

The man traveling in the back of the ambulance which was running at a high speed along the streets of Baltimore that morning in 2008 had no business to be alive.By everything that was reasonable,and there were plenty of such things before,he should have been very dead indeed.But he wasn’t.As the people in the hospital pointed out after they had examined him,he was only slightly hurt.Yet he had just fallen 150 feet down a hotel lift shaft(电梯通道)!

Unknown to the man,two things had occurred which were to affect his life that day.On the thirteenth f1oor of the hotel, somebody had carelessly left the lift gate open.Down in the basement, a pipe had burst and,it had flooded the bottom of the lift shaft to a depth of two feet.

Modern lifts have all sorts of fail-safe system to prevent accidents,but this was ancient equipment unreliable,slow,dangerous,and suitable material to recycle.

The man had plenty of things to occupy his mind that morning. He had overslept. The hotel had forgotten to call him and now he was late for an important business appointment. He dressed quickly, shaved hurriedly, took hold of his briefcase and hurried off down the hotel corridor.

Good! The life gate was open. The life must be there. He need not press the button and wait while the large, clumsy life made its way upwards. Without looking or thinking, he stepped out into space. The lift cage was, in fact, one floor above him on the fourteenth. The would which he had walked was a narrow space of not very fresh sir, ending 150 feet below in two feet of dirty water.

The man fell, making his journey to the ground at a speed he had never dreamed of. Confused patterns, a rush of air, time enough to be afraid, split-second thoughts of death, then-crash!

Perhaps this gave him the record for some sort of high-diving act. No doubt in future he always looked before he jumped. Certainly be learnt that this was no way to save time. The experts said that those two feet of water had saved his life.

59.What do we learn about the man?

A.He fell from the 13th floor.       B.He was hit by an ambulance.

C.He got caught in a serious flood.  D.He made a record for high-diving.

60.By “……had no business to be alive”,the writer means that the man_______.

A.had missed his business appointment      B.was alive with excitement

C.was alive and this was surprising       D.didn’t do any business

61.Tile lift did not have a fail-safe system because it was____________.

A.narrow     B.slow  C.1arge D.old

62.Which of the following was NOT the cause of the accident?

A.A pipe burst.                  B.The man overslept

C.The hotel forgot to call him      D.Someone left the lift door open

问答题 简答题