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如图所示为一种蓄水箱的人工放水装置,AOB是以O点为转轴的轻质杠杆,AO呈水平状态,A、O两点间的距离为40cm.B、O两点间的水平距离为10cm,竖直距离为7cm,K是一轻质、横截面积为100cm2的盖板(恰好堵住出水口),它通过细绳与杠杆的A端相连,在水箱右侧水平地面上,重为600N的人通过滑轮组拉动系在B点呈竖直状态的绳子,可以控制出水口上盖板,若水箱水深为50cm,当盖板恰好要被拉起时,水平地面对人的支持力为490N,人对绳子的拉力为F1,绳子对B点的拉力为F2,滑轮组的机械效率为η,盖板的厚度、绳重与滑轮间的摩擦均可忽略不计,人对绳的拉力与人所受重力在同一直线上(g取10N/kg)求:

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     What is a hurricane?

    You may already know that hurricanes are storms that can cause devastating(毁灭性的)waves, wind, and rain. They happen during “Hurricane Season,” which is from June 1st until November 30th in the Atlantic Ocean and from May 15th until November 30th in the Pacific Ocean.

Categories(类别)of Hurricanes

    There are five categories of hurricanes, which are based on wind speeds. The categories help people know how much damage(危害)a hurricane may cause because the greater the wind speed, the more dangerous the storm is. For example, a Category 5 hurricane has winds over 155 miles per hour, which are strong enough to destroy the buildings along the shorelines completely.

Hurricane Dangers

    When a hurricane happens together with heavy rainfall, there can be a devastating flood. The centre of a hurricane is called the eye. While most of a hurricane has dangerously strong winds, the eye is actually(事实上)a calm area in the storm. When the eye of a hurricane passes over land, people might think that it’s over, but before long the wind and rain increase again as the second part of the hurricane moves through.

Hurricane Safety Tips

    There is no way to stop a hurricane or make it change direction, so if you ever find yourself in the path of a hurricane, be sure to remember the following safety tips.

    Be sure you have a radio, batteries, fresh drinking water, and enough food.

    Tell neighbours, friends, and family members your emergency plans. Tell them where you'll go if you need to leave your home.

    If you live near the ocean, leave your home and travel to a safe place. You could stay with a friend or a family member, in a hotel or in an emergency shelter area.

    Stay inside during the storm. You could be seriously injured (受伤) if you go outside.