问题 多选题

滑块以某初速度从固定的粗糙斜面底端向上运动,然后又滑回到斜面底端,若滑块向上运动的位移中点为A,取斜面底端重力势能为零,则滑块(  )

A.上滑过程机械能减小,下滑过程机械能增大

B.上滑过程机械能减小,下滑过程机械能也减小

C.上滑至A点时动能大于势能

D.下滑至A点时动能大于势能

答案

A、B、除重力外其余力做的功等于物体机械能的变化量,物体滑动过程中,支持力不做功,摩擦力一直做负功,故机械能不断减小,故A错误,B正确;

C、设最高点为B点,物体上滑过程中:EkA>EkB,即mg

h
2
+
1
2
m
v2A
>mgh,故
1
2
m
v2A
>mg
h
2
,即上滑时A点的动能大于势能,故C正确;

D、下滑过程中,机械能不断减小,有:EkB>EkA,即mgh>mg

h
2
+
1
2
m
v2A
,故mg
h
2
1
2
m
v2A
,即下滑时在A点势能大于动能,故D错误;

故选BC.

阅读理解

阅读理解。

     During the cold winter days you may find it hard to wake up in the morning. It's warm and cozy

(舒适的) in your bed.

     In winter, many animals like to sleep, too. But their sleeps can be very long and deep. This kind of

sleep is called hibernation (冬眠). During this kind of sleep, their temperatures go down, and hearts beat

(跳动) slowly. They take very small breaths.

     Animals can hibernate for part or all of the winter. It's hard for them to find things to eat. When they

go to sleep, they don't need much food.

     Bears, hedgehogs (刺猬) and chipmunks (花栗鼠) go to sleep in their homes. Frogs and snakes

hibernate, too. Frogs move into the earth under water to keep themselves warm.

     Before they go to sleep, they eat a lot to get fat (脂肪). Some also keep food in their homes to eat later.

Bears are the biggest animals that hibernate. They often sleep in caves (山洞).

     Bears must eat a lot of things like nuts before going to sleep for a long time. They eat as much as 300

apples (that's the same as 60 hamburgers) each day for many weeks. They can eat for 20 hours in a day!

     Bears can go into a very deep sleep. It can last from three months to half a year. During that time they

do not eat, drink or go to the toilet. A mother bear can give birth to babies during her hibernation.

     Other animals sleep less. Sometimes they wake up to eat, or take a walk. Then they go to sleep again.

根据短文内容判断正(T)误(F):

( )1. Hibernation is a the long and deep sleep of animals in winter.

( )2. During hibernation, animals don't breathe at all.

( )3. Bears eat lots of food before they hibernate in winter.

( )4. Bears don't go to the toilet during their hibernation.

( )5. Frogs hibernate in the Earth under water.

( )6. Mother bears can't give birth to babies during hibernation.

( )7. We can infer that fat is an important food for animals during hibernation.

( )8. Bears, hedgehogs, chipmunks, pigs and frogs hibernate.

解答题