问题 多选题

为了求出楼房高度,让一石子从楼顶自由下落,空气阻力不计,已知重力加速度的数值,测出下列哪个物理量的值就能计算出楼房高度(  )

A.石子开始下落1s的位移

B.石子最后下落1s的位移

C.石子开始下落1m的时间

D.石子最后下落1m的时间

答案

A、已知石块下落第一米的时间,无法知道整个过程下落的时间,故无法求出石块下落的高度,故A错误;

B、知道石块落地前最后1s的位移x,可知最后1s的平均速度

.
v
=
x
1
(m/s),根据vt=v0+gt可知小球落地时的速度v=
.
v
+
g
1
2
=
x
1
+
1
2
g(m),再根据v2=2gh求解总高度;故B正确;

C、石块下落1米位移的时间t=

2h
g
=
2×1
10
=
5
5
s,与运动总时间无关,故C错误;

D、已知石块通过最后1m的时间t,则最后1m的平均速度

.
v
=
1
t
(m/s);

可知石块落地时的速度v=

.
v
+g
t
2
=
1
t
+
gt
2
(m/s),再根据v2=2gh求解总高度;故D正确;

故选BD.

阅读理解

In nature, you may see many insects (昆虫)and animals of different colors. Have you ever wondered why?

Locusts are super delicious food for birds,but it is not always an easy job for birds to catch them.It is neither because locusts are good jumpers or runners nor because they are smarter than their enemies. The reason is that the colors of locusts change with the colors of crops(庄稼). When crops are young,locusts look green. But when autumn comes, locusts change to the yellow and brown color as crops do.

Brown bears,tigers and other animals move quietly through forests. They can’t be seen easily by their enemies. This is because they have colors similar to the trees. For the same reason, polar bears that live on a land of snow and ice are white. Butterflies and bees living among the flowers are colorful like flowers. Soil insects are mostly dark-colored and they live under the soil(土壤) in a dark and wet environment. However, insects with colors different from plants can easily be found and eaten by others. So in order to survive,they have to hide themselves in the daytime and appear only at night.

Have you ever noticed some even stranger acts? The ink fish in the sea can send out some very black ink when it faces danger. As the ink spreads over , its enemies will find themselves suddenly in a dark world. At the same time the ink fish immediately swims away. That is how it keeps itself safe though it is not strong at all.

小题1:It is not easy for birds to catch locusts because____________.

A.locusts are good runners

B.locusts can jump very high

C.locusts are smarter than birds

D.locusts can change their color小题2:According to the passage, most soil insects_________

A.do not have bright colors

B.change their colors with the soil

C.do not live in a dark and wet environment

D.Eat colorful insects like butterflies and bees小题3:What’s the meaning of the underlined word”survive” in the passage?

A.繁殖

B.产卵

C.觅食

D.生存小题4:The ink fish_______so that its enemies will not see it?

A.changes its color

B.makes the water around it black

C.appears only at night

D.hides itself in the sea rocks小题5:What is the main idea of this passage?

A.Some insects and animals have different skills.

B.Some insects and animals are clever than we think.

C.Some insects and animals use colors to protect themselves.

D.Some insects and animals can get used to the environment easily.

选择题