题目

如图所示，固定在地面上的光滑圆弧轨道AB、EF，他们的圆心角均为90°，半径均为R。一质量为m 、上表面长也为R的小车静止在光滑水平面CD上，小车上表面与轨道AB、EF的末端B、E相切。一质量为m的物体（大小不计）从轨道AB的A点由静止下滑，由末端B滑上小车，小车在摩擦力的作用下向右运动。当小车右端与壁DE刚接触时，物体m恰好滑动到小车右端相对于小车静止，同时小车与DE相碰后立即停止运动但不粘连，物体则继续滑上圆弧轨道EF，以后又滑下来20、Bicycles are very popular around the world today．People ride their bicycles for exercise and enjoyment．In some places，people use a bicycle to get to work．In other places，bicycle riding is a very popular exercise to people who live in cities．But who invented the first bicycle? In 1791，a Frenchman named Comte de Sivrac invented and owned the first bicycle．Mrs. Sivrac rode（骑）the bicycle in Paris．The handles（把手）and the seat were wooden．This bicycle was very difficult to move．A rider even had to pick up the front wheel to change direction．Finally，the bicycle had no brakes for stopping or pedals for the feet! Riding a bicycle was a great risk． In 1817，a German named Baron von Drais de Sauerbrun made the first bicycle better．The seat became more comfortable．The wheel could now change direction．His ride in the forest took only one hour instead of three hours on foot，which surprised people at that time． Sauerbrun brought this kind of bicycle from Germany to France．Then Denis Johnson，an Englishman，made a bicycle for women．It had space for their dresses to hang down．But these bicycles still had no brakes or pedals，and riders often got hurt．These bicycles and the people who rode them were not very popular． It took another forty-five years for the bicycle to become popular．More than 100 years later，bicycle riding is more popular than ever．In fact，in India and China，there are still many more bicycles than ears． 1．Which of the following best describes the first bicycle? A．Very popular．                                    B．Easy to carry．       C．Ugly-looking．                                    D．Difficult to ride． 2．Why was it a risk to ride the first bicycle? A．Because it had no pedals for the feet． B．Because it was difficult to control（控制）． C．Because people would laugh at you． D．Because handles and the seat were wooden． 3．Which of the following was not improved by Sauerbrun to the first bicycle? A．Brakes were added to it． B．It was made to go faster． C．The wheel could now change direction． D．The seat was made more comfortable． 4．What’s the major difference between Denis Johnson’s bicycle and the earlier ones 7 A．It was more comfortable．                          B．It was faster and safer． C．It had space for the rider's dress．               D．It had metal handles and seat． 5．The best title（题目）for the passage would be                 ． A．Bicycles Around the World                          B．How to Ride a Bicycle C．The Greatest Invention                                D．The First Bicycle