在动摩擦因数μ=0.2的水平面上有一个质量为m=1kg的小球，小球与水平轻弹簧及与竖直方向成θ=45°角的不可伸长的轻绳一端相连，如图所示，此时小球处于静止平衡状态，且水平面对小球的弹力恰好为零，当剪断轻绳的瞬间，取g=10m/s2，求：
（1）此时轻弹簧的弹力大小
（2）小球的加速度大小和方向．
（1）水平面对小球的弹力为零，小球在绳没有断时受到绳的拉力F、重力mg和弹簧的弹力T作用而处于平衡状态，如图所示：
由平衡条件得：
竖直方向：Fcosθ=mg
水平方向：Fsinθ=T
解得：T=mgtanθ=10N
当剪断轻绳瞬间弹簧的弹力大小不变，仍为10N；
（2）剪断轻绳后小球在竖直方向仍平衡，
水平面支持力与重力平衡：N=mg
由牛顿第二定律得：T-μN=ma
解得：a=8m/s2 方向向左．
答：（1）此时轻弹簧的弹力大小为10N；
（2）小球的加速度大小为8m/s2，方向向左．
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| Jody was ten years old when he decided he needed a job. He thought it might be 1 to raise (养) worms. He could sell them to farmers and people who fished. So in 2 , he bought many worms. But that winter the cold weather killed all the worms because he had not 3 them in a warm place. The next spring Jody 4 again. He bought more worms, which he took good care of. When winter came, he took them inside 5 they would stay warm. Many people bought his worms. One day when Jody was twelve, he got a letter. It was frome state (州) of New York. The letter said, "Everyone who 6 things has to pay taxs (税)!" Jody made only one dollar selling worms. But he still 7 to pay part of that money to the state. He told many people in his town what had 8 . Soon some people from a television station 9 with Jody. Many people saw it and they began to write letters to the state. The letters now said that the law was 10 . Finally the law was changed. Children like Jody can now sell things without paying money to the state. | ||||
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| We all know that certain oils are healthier than others,but your oil health goes beyond just the type. The health of your oil can be related to how you use it too. Each type of oil has what is called a "smoke point". The smoke point is the specific temperature at which the oil starts to break down,or in more technical terms,its molecular structure begins to change. These molecular changes result in changes in flavor,as well as changes in nutritional value-specifically, the nutritional value of the oil starts to reduce,changing what once may have been considered an especially healthy oil,like olive,into one that is unhealthy. The higher an oil's smoke point is,the higher the temperature of the oil can be.As a result,each type of oil should be used for the cooking method that is most appropriate to its individual smoke point and heat tolerance.Here is a quick guide for the next time you reach for your favorite oil.
Refined oils tend to have much higher smoke points than when they are unrefined.They also differ in nutrition and flavor.Unrefined oils are more nutritious and they tend to be much richer in flavor. When it comes to extremely high heat cooking,always choose oils which are refined. 1. What determines whether an oil is healthy or not? A. Whether it's refined or not. B. Its smoke point. C. Its molecule. D. How you use it. 2. Which of the following might be a better choice to make a healthy salad? A. Refined walnut oil. B. Safflower oil. C. Unrefined olive oil. D. Sunflower oil. 3. Healthy oil can become unhealthy if________ according to the passage. A. you keep it for too long B. it is heated well beyond its smoke point C. it is refined D. its flavor is changed 4. The author wants to tell readers that________. A. oils are related to health B. oils' value depends on their smoke points C. "certain oils are healthier" is relative D. oils' flavor and nutrition are connected |