问题 计算题

某同学做拍打篮球的游戏,要控制篮球,使其重心在距地面高度为h=0.9m的范围内做竖直方向上的往复运动,如图所示。每次要在最高点时用手开始击打篮球,手与球作用一段距离后分开,球落地反弹。已知球反弹的速度v2的大小是落地速度v1大小的4/5,反弹后恰好达到最高点,球与地面的作用时间为t=0.1s,篮球的质量m=0.5kg,半径为R=0.1m,若地面对球的作用力可视为恒力,篮球与地面碰撞时认为重心不变,忽略空气阻力和篮球的转动。

求(1)球反弹的速度v2.(2)地面对球的作用力F(g取10m/s2

答案

(1)4m/s   (2)50N

题目分析:(1)从球反弹后至达最高点,此过程

可得

(2)设球与地面接触时加速度为a,

由题知

球下落刚触地至反弹后刚离开地面过程,设向上为正方向

球触地过程受力如图,

由牛顿第二定律得F-mg=ma

代入数据解得F=ma+mg=50N

点评:本题中①篮球不能看成质点,②注意速度的正负,③计算地面对球的作用力是不要忽略了重力。

选择题
阅读理解

Bells sound. Lighted messages appear. Men and women work at computers. They talk on the telephone. At times they shout and run around.

This noisy place is a stock exchange (证券交易所). Here expert salespeople called brokers buy and sell shares(股份) of companies. The shares are known as stocks. People who own stock in a company own part of that company.

People pay brokers to buy and sell stocks for them. If a company earns money, its stock increases in value. If the company does not earn money, the stock decreases in value. Brokers and investors(投资者)carefully watch for any changes on the Big Board. That is the name given to a list of stocks sold on the New York Stock Exchange.

Investors and brokers watch the Big Board to see if the stock market is a bull market or a bear market. In a bear market, prices go down. In a bull market, prices go up.

Investors in a bear market promise to sell a stock in the future at a set price. But the investor does not own the stock yet. He or she waits to buy it when the price drops.

The meaning of a bear market is thought to come from an old story about a man who sold the skin of a bear before he caught the bear. An English dictionary of the sixteen hundreds said, to sell a bear is to sell what one has not.

Word experts dispute the beginnings of the word bull in the stock market. But some say it came from the long connection of the two animals – bulls and bears – in sports that were popular years ago in England.

Investors always care about the possibility of a company failing. In the modern world, a company that does not earn enough profit (利润)is said to go belly up. A company that goes belly up dies like a fish. Fish turn over on their backs when they die. So they are stomach, or belly up.

Stock market investors do not want that to happen to a company. They want a company whose stock they own to earn more profit than expected. This would sharply increase the value of the stock. Investors are hoping for a windfall(横财).

72. The first paragraph is written for the purpose of ________.

A. making readers interested in buying stocks

B. telling readers the place is so busy

C. telling readers people in the stock exchange are busy

D. attracting readers’ attention to the topic of the passage

73. A fresher of a stock market probably turns to a (n) _______ for advice.

A. company                                 B. investor

C. broker                                   D. word expert

74. John bought his stock from a company which has gone belly up this year. We can infer _________ .

A. John would lose money this year

B. John would earn money this year

C. the stock market is a bear market

D. the stock market is a bull market

75. The reason why investors pay attention to the company whose stock they own is that _______ .

A. the company belongs to them

B. the company earns much money

C. they work for the company

D. they are financially connected with the company