问题 问答题

如图9-4-7所示,一个半径为R的圆弧形光滑轨道竖直放置.现有两质点P、Q.Q从圆弧轨道的圆心处自由落下(空气阻力不计),P同时从非常接近O点的B点无初速释放.问:P与Q谁先到达O点?

图9-4-7

答案

Q先到达

Q从圆心处自由下落,设到达O点时间为t1,则R=gt12,所以t1=   ①

P从非常接近O点的B点无初速释放,相当于单摆运动,圆轨道对P球的支持力代替了细线的拉力,且偏角可以认为不超过10°,即可视为简谐运动.它到达O点所需时间设为t2,有t2=,T=2π

所以t2=                                                           ②

比较①②式可知t1<t2,所以Q质点先到达O点.

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Captain Goodfellow

Do your children enjoy interesting stories, funny games, and exciting dances? Captain Goodfellow will be ready to teach all these things to children of all ages at the City Theatres on Saturday morning at 10:00 Free.

Walking Tour of the Town

Forget your worries on Saturday morning. Take a beautiful walk and learn about local history. Meet at the front entrance of City Hall at 9:30. Wear comfortable shoes!

Films at the Museum

Two European films will be shown on Saturday afternoon at the Museum Theatre. See Broken Window at 1:30. The workers will be at 3:45. For further information, call 4987898. International Picnic

Are you tired of eating the same kind of food every day? Come to Central Park on Saturday and enjoy food from all over the world. Delicious and not expensive. Noon to 5:00 p.m.

Take me out to the Ballgame

It’s October, and tonight is your last chance to see the Redbirds this year. Get your tickets

at the gate. It might be cold… don’t forget sweaters and jackets.

Do you want to hear “The Zoo”?   

“The Zoo”, a popular rock group from Australia, will give their first US. Concert tomorrow night at 8 Rose Hall, City College.

小题1:You can probably eat Chinese, Italian and Arab food _________.

A. at Central Park on Saturday                            B. at the front entrance of City Hall

  at the ballgame                                     D. at 5:00 p.m.

小题2:If you are going on the Walking Tour, don’t forget _________.

A. your beautiful walk                                 B. your comfortable shoes

  your worries                                               D. your learning about local history

小题3:The Redbirds ballgame _________.

A. is in the afternoon              B. is at the gate      might be cold         D. is outside

小题4:Which of the following is NOT true?

A. “The Zoo” is an Australian rock group.

B. “The Zoo” are going to meet their American audience for the last time.

  “The Zoo” will appear at 8:00 p.m.

D. “The Zoo” will perform in front of college students.