Forces other than damaging winds are also at work inside tornadoes. Sometimes, as the twisting funnel passes over a house, the walls and ceiling burst apart as if a bomb had gone off inside. This explosion is caused by the low air pressure at the center of a tornado.
The pressure at the center of a tornado is usually 13 pounds per square inch. However, inside the house the air pressure is normal, a bout 15 pounds per square inch. The difference of 2 pounds per square inch between the inside and outside pressure may not seem like much. But suppose a tornado funnel passes over a small building that measures 20 by 10 feet. On each square inch of the building, there is 2 pounds of pressure from the inside that is not balanced by air pres sure outside the building. On the ceiling, that adds up to an unbalanced pressure of 57 600 pounds. The pressure on the four walls adds up to 172 800 pounds.
If windows are open in the building, some of the inside air will rush out th
A. a house
B. a tornado
C. a bomb
D. an explosion
Americans with small families own a small car or a large one. If both parents are working, they usually have two cars. When the family is large, one of the cars is sold and they will buy a van(房车).
A small car can hold four persons and a large car can hold six persons but it is very crowded. A van hold seven persons easily, so a family three children could ask their grandparents to go on a holiday travel. They could all travel together.
Mr. Hagen and his wife had a third child last year. This made them sell a second car and buy a van. Their sixth and seventh seat are used to put other things, for a family of five must carry many suitcases(衣箱)when they travel. When they arrive at their grandparents’ home, the suitcases are brought into the two seats can then carry the grandparents.
Americans call vans motor homes. A motor home is always used for holidays. When a family is traveling to the mountains or to the seaside, they can live in their motor home
A. a motor car
B. a motor home
C. a motor bike
D. a big truck
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