问题 多选题

在一粗糙的斜面上放置一正方形的箱子,其内部刚好放入一质量一定的金属球,如图所示,现在从斜面顶部释放箱子,在其加速下滑过程中,下列关于球对箱子的作用力,说法正确的是(  )

A.球对箱子a面有压力

B.球对箱子d面有压力

C.球对箱子c面有压力

D.球对箱子b面有压力

答案

设箱子的质量为M,球的质量为m,斜面与箱子间的动摩擦因数为μ,以整体为研究对象,根据牛顿第二定律得

    (M+m)gsinα-μ(M+m)gcosα=(M+m)a,得a=g(sinαμcosα),方向沿斜面向下.

再以小球为研究对象,设箱子在垂直于斜面方向对小球的作用力方向向上,大小为F1,平行于方向对小球的作用力方向向下,大小为F2,根据牛顿第二定律得

垂直于斜面方向:F1=mgcosα>0,说明箱子对小球垂直斜面方向的力垂直于斜面向上,则箱子c面对小球有支持力.

平行于斜面方向:mgsinα-F2=ma,得到F2=μmgcosα<0,说明箱子平行于斜面方向对小球的力方向沿斜面向上,则箱子b面对小球有力.

根据牛顿第三定律得知,球对箱子c面和b面有压力.

故选CD

填空题
阅读理解

When buying from a dealer, the law says that a car must be:

As described:

This includes the history of the car as well as its specification.For example, if the dealer described the car as previously having “one careful lady owner”, it shouldn’t turn out to have had several previous “boy racer” owners.

Of satisfactory quality:

It must meet the standard that a reasonable person would regard as acceptable and be free from any quality problem.Also, bear in mind that a second-hand car will have a slightly different definition of what is considered “satisfactory, because there’s certainly an element of wear and tear.

Fit for the purpose:

It must be reasonable fit for any normal purpose and this includes any purpose that you specify to the seller.

●If any of the above is violated, then in theory, you may have the right to reject the vehicle and get your money back if you’re reasonably quick. Alternatively, the dealer might offer to replace or repair the car; reduce the price of offering a partial refund(退款).Once you’ve informed the dealer that you wish to reject the car , you must stop using the vehicle.

●If the rejection is not accepted, then it’s up to you to prove your case. You’ll need to pay for an independent assessment of the car and sue(打官司,起诉)for damages. If you do choose a repair, insist the dealer provide you with a hire car or pay any reasonable traveling expenses thus produced while your new car is in the garage.

●If the car is new, it’s likely that the claim will be too high to be fought. Using the small claims procedure to you may have to pay for legal representation. All this can be pretty daunting and expensive. You need to weigh up the pros and cons before rejecting a car. Would a repair do just as well? Selecting a dealer who offers a clear exchange policy may help.

小题1:According to the text, what is essential after you inform the dealer of your wish to reject the car?

A.Stopping using it

B.Demanding traveling expenses.

C.Suing for damages.

D.Proving your case.小题2:If the dealer offers to repair the car you have bought, which of the following is acceptable?

A.Asking the dealer to provide you with repair tool sets

B.Asking the dealer to provide you with a hire car

C.Asking the dealer to pay all of your traveling expenses.

D.Asking the dealer to return all your money.小题3:Which of the following words has the closest meaning to the underlined word “daunting” in

the last paragraph?

A.Different.

B.Discouraging.

C.Effective.

D.Convenient.小题4:What does the writer want to say in the last paragraph?

A.Buyers should select a dealer that offers to repair the car.

B.Buyers should never reject a new car.

C.It’s necessary that a new car should be repaired.

D.Rejecting a new car is not necessarily the best choice.小题5:After reading the passage, you will be better at         .

A.choosing a car

B.making a claim

C.dealing with car sellers

D.suing for damages