问题 选择题

如图所示的装置中,木块与水平桌面间的接触是光滑的,子弹沿水平方向射入木块(子弹射入木块过程时间极短,可认为弹簧长度不发生变化)后留在木块内,将弹簧压缩到最短。对子弹、木块和弹簧组成的系统,下列说法正确的是   (   )

A.在子弹射入的过程中动量守恒、机械能不守恒

B.在子弹射入的过程中动量不守恒、机械能守恒

C.在从子弹开始射入木块到弹簧压缩至最短的整个过程中动量不守恒、机械能不守恒

D.在从子弹开始射入木块到弹簧压缩至最短的整个过程中动量守恒、机械能也守恒

答案

AC

在子弹射入的过程中系统合外力为零动量守恒、有内能产生机械能不守恒,A对,B错。

在从子弹开始射入木块到弹簧压缩至最短的整个过程中系统合外力不为零动量不守恒、机械能有损失不守恒,C对,D错,所以选AC

阅读理解

Many animals hide(躲藏) to stay alive. They hide in many ways. Some animals hide in leaves; some animals hide in snow. Usually their colors or shapes(形状) help protect them. It’s hard to see an Arctic bear in snow. Its white fur helps protect it. Chameleons(变色龙) can hide by changing colors. Their skin turns the same color as what is around them. Some chameleons can change colors in five minutes.

Many animals try to look bigger than they are to stay alive. Some animals can make other animals think they are very big. If they look very big, animals that are looking for food will run away. Bears can walk on two legs. They look very, very scary(可怕的). Some animals will run away from them.

Many animals try to stay around other animals. This helps them stay alive. Zebras stay close together so that they can help each other look out of danger. Seeing many stripes(条纹) at once can also confuse(使混乱) animals that want to eat them. Some birds stay in a circle, each toward a different direction. In this way they can also help each other stay alive by looking around for animals that may want to eat one of them.

小题1:Animals have ways to stay alive. Which animal try to look bigger than they are to stay alive in the passage?

A.Zebras.

B.Chameleons.

C.Birds.

D.Bears.小题2:Why do animals hide?

A.Because they like to play hide-and-seek.

B.Because they are shy.

C.Because they want to catch other.

D.Because they want to protect themselves.小题3:How do Chameleons hide?

A.They change colors.

B.They change shapes.

C.They hide in snow.

D.They hide in leaves.小题4:How do zebras hide?

A.Become good friends.

B.Talk to each other.

C.Help each other look out for danger.

D.Eat and play together.小题5:Why do animals try to look bigger than they are?

A.They want other animals to run away from them.

B.They want other animals to follow them.

C.They want other animals to see them.

D.They want other animals to like them.

单项选择题