问题 选择题

如图所示,带正电的物块A放在足够长的不带电小车B上,两者均保持静止,处在垂直纸面向里的匀强磁场中,在t=0时用水平恒力F向右拉小车B,t=t1时A相对B开始滑动,t=t2以后B作匀加速直线运动,已知地面光滑、AB间粗糙,A带电荷量保持不变,则关于A、B的图象,下图大致正确的是(  )

A                 B                 C                  D

答案

答案:C

题目分析:在t=t1之前物体A与小车共同做匀加速直线运动,根据牛顿第二定律得:F=(m+M)a,所以小车与物块的速度随时间均匀增大; 对物块A根据牛顿第二定律有:f=ma.即静摩擦力提供其加速度,根据左手定则判断出物块A所受洛伦兹力方向竖直向上,物块所受的洛伦兹力qvB逐渐增 大,由于物体A竖直方向受力平衡,所以A与B之间的压力减小,即它们间的最大静摩擦力减小,当两物体A、B之间的最大静摩擦力不能提供物块A原来的加速度 a时,A、B发生相对滑动.

当物块A与小车B之间的最大静摩擦力不能提供物块A原来的加速度a时,物块与小车发生了相对滑动,此时物块A受到向右的滑动摩擦力f1=μFN虽然小于刚才的静摩擦力,但是滑动摩擦力的方向仍然向右,物块A仍然加速运动,物块所受向上的洛伦兹力qvB逐渐增大,由于物体A竖直方向受力平衡,物块A与小车B之间的压力减小,所以向右的滑动摩擦力也减小,即物块A的加速度在减小,直到t2时刻加速度减小到零,最后做匀速直线运动,在速度-时间图象中物块A的斜率逐渐减小到零;故AD错误.

当物块与小车发生了相对滑动,小车受到物块施加的向左的滑动摩擦力一直减小,由于水平恒力F是定值,所以小车受到向右的合力一直增大,即小车的加速度逐渐增大,在速度-时间图象中,小车的图象斜率从t1时刻开始增大,直到t2时刻,两物体间无摩擦力时,小车水平方向受到的合力F保持不变,即小车做匀加速直线运动,故B错误,C正确.

故选:C

点评:对于这类问题正确受力分析结合牛顿第二定律判断物体的加速度变化,再结合运动学关系式判断物体的速度变化情况.

阅读理解

On December 18, 1620, passengers on the British ship Mayflower came on shore at modern – day Plymouth, Massachusetts, to begin their new settlement, Plymouth Colony.

The famous Mayflower story began in 1606, when a group of reform-minded people in England built their own church, which was separate from the state – supported Church of England. Because of this, they were forced to leave the country and settle in Netherlands. After 12 years of struggling to adapt and make a hard living, the group sought financial backing from some London merchants to set up a colony in America.

On Septmeber 6,1620, one hundred and two passengers – called Pilgrims (朝圣者) by William Bradford, a passenger who would become the first governor of Plymouth Colony – crowded on the Mayflower to begin the long, hard journey to a new life in the New World. On November 11,1620, the Mayflower landed at Provincetown Harbor. Before going on shore, 41 male passengers – heads of families, single men and three male servants – signed the famous Mayflower contract, agreeing to form a government chosen by common election and to obey all laws made for the good of the colony.

Over the next month, several small leading teams were sent on shore to collect firewood and hunt for a good place to build a settlement. Around December 10, one of these groups found a harbor they liked on the western side of Cape Cod Bay. They returned to the Mayflower to tell the other passengers, but bad weather prevented then reaching the harbor until December 16. Two days later, the first group of Pilgrims went on shore. After exploring the region, the settlers chose a cleared area once controlled by members of a local Native American tribe (部落). The tribe had abandoned the village several years earlier, after an outbreak of European disease.

That winter of 1620 – 1621 was really hard, as the Pilgrims struggled to build their settlement, find food and take care of the sick. By spring, 50 of the original 102 Mayflower passengers were dead. The remaining settlers made contact with returning members of the Native American tribe and in March they signed a peace treaty (条约) with the tribe chief. In the spring time, helped by the locals, the Pilgrims were able to plant crops – especially corn and beans – that were necessary to their survival.

On April 5,1621, the Mayflower and its crew left Plymouth to return to England. Over the next several decades, more and more settlers made the voyage across the Atlantic to Plymouth, which gradually grew into a successful shipbuilding and fishing center.

1.A group of English were forced to leave their country in 1606 because          .

A.their belief was different from the official Church

B.they built their own churches in a foreign country

C.they lacked financial support from the government

D.they were ambitious to build the colony in America

2.How many days did the sea journey to Provincetown Harbor last?          .

A.103   B.96     C.66     D.35

3.Several leading teams were sent in order to        .

A.hunt for food for the passengers     B.control local Native Americans

C.build a settlement for the passengers      D.explore the region for their settlement

4.From the passage we know that         between 1620 – 1621.

A.52 Mayflower passengers died of some illnesses   

B.Pilgrims had a good relationship with the locals

C.Plymouth turned a main center for shipbuilding   

D.Pilgrims tried to plant corns and beans in winter

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