问题 计算题

(19分)如图12所示,质量M=1.0kg的木块随传送带一起以v=2.0m/s的速度向左匀速运动,木块与传送带间的动摩擦因数μ=0.50。当木块运动至最左端A点时,一颗质量为m=20g的子弹以v0=3.0×102m/s水平向右的速度击穿木块,穿出时子弹速度v1=50m/s。设传送带的速度恒定,子弹击穿木块的时间极短,且不计木块质量变化,g=10m/s2。求:

(1)在被子弹击穿后,木块向右运动距A点的最大距离;

(2)子弹击穿木块过程中产生的内能;

(3)从子弹击穿木块到最终木块相对传送带静止的过程中,木块与传送带间由于摩擦产生的内能。(AB间距离足够长)

答案

(1)(2)(3)12.5J

(1)设木块被子弹击穿时的速度为u,子弹击穿木块过程动量守恒

解得     ………………………………(2分 

设子弹穿出木块后,木块向右做匀减速运动的加速度为a,根据牛顿第二定律μmg=ma 解得            …………………………………………(2分)木块向右运动到离A点最远时,速度为零,设木块向右移动最大距离为s1

  解得        ………………………………………(2分)    (2)根据能量守恒定律可知子弹射穿木块过程中产生的内能为

E=    …………………………(3分) 解得      …………………………………………………(1分)(3)设木块向右运动至速度减为零所用时间为t1,然后再向左做加速运动,经时间t2与传送带达到相对静止,木块向左移动的距离为s2。根据运动学公式

解得     ………………………………………(2分) 

t1=="0.60s," t2==0.40s   ………………………………………(1分)木块向右减速运动的过程中相对传送带的位移为

, 产生的内能 ……………………………………(2分)木块向左加速运动的过程中相对传送带的位移为m,

产生的内能   ……………………………………(2分)

所以整个过程中木块与传送带摩擦产生的内能

    …………………………………………………(2分)

本题考查的是动量守恒定律及能量守恒的综合力学问题,先根据动量守恒及牛顿第二定律解出木块向右移动最大距离;再根据能量守恒解出产生的内能;再根据匀变速运动的规律及摩擦力做功计算出产生的内能;

阅读理解

阅读理解。

     Most sharks are dangerous. The largest kind of sharks-the whale shark-has small teeth and is quite harmless

 to people. However, blue sharks, tiger sharks, white sharks and hammerheads are enemies of man. Man-eaters

are always a danger to swimmers in shark waters. Sometimes, though, men have used strange methods when

they're suddenly faced with a shark.

     During world War Ⅱ, soldiers and sailors whose boats or planes were destroyed drifted (漂流) helplessly

on the ocean in small rafts (筏子). While waiting to be saved, the men had to struggle to stay alive and were

often attacked by sharks. In some cases, they had few weapons to protect themselves with. They found that

just splashing (溅水) water seemed to help keep the sharks away.

     One sailor was swimming for his life in the Atlantic Ocean after his boat had been blown up. He saw a

shark swimming towards him. He hit the shark with his bare fist, and that drove it off.

     Once, in Australia, a fisherman was fishing after sunset. He hooked a big fish and drew it towards the shore. His line broke when the fish was only a few yards from the shore. The fisherman waded (涉水) into the water

to try to catch it with his hands. In the dim light, it looked like the kind of fish that could be dealt with without

danger.

     He put his arms around the fish and wrestled with it. Dragging it to the beach turned out to be harder than

he had expected, but at last he pulled it in. When he turned a light on it, he was amazed. He had caught a shark.

It was small for a shark, but it was the man-eating type and was about as long as a tall man. In a word, the

best way to deal with sharks is to keep far away from them.

1. The shark that is least dangerous to man is the _______.

A. white shark

B. whale shark

C. tiger shark

D. hammerhead shark

2. This story tells you that sharks are usually ________.

A. friendly

B. small

C. afraid of people

D. dangerous

3. After the Australian fisherman found he had caught a shark, he was ________.

A. surprised

B. frightened

C. unhappy

D. angry

4. This story tells you that the best way to handle sharks is to ________.

A. hit them with your fist

B. keep far away from them

C. splash water

D. wrestle with them

5. The passage mainly tells us something ________.

A. about fishing sharks

B. about sharks

C. how to eat sharks

D. that sharks are man's enemies

单项选择题