如图所示，绳OC与竖直方向30°角，O为质量不计的光滑的滑轮，用一根绳连接物体A和B，已知物体B重1000N，物体A重400N，物块A空中静止，物体B在地面上静止．求：
（1）OC绳的拉力为多大？
（2）物体B所受地面的摩擦力和支持力分别为多大？
（1）由于物体A保持静止，故：T=GA=400N；
对滑轮受力分析，受三个拉力，如图所示：
根据平衡条件，有：T0C=2TCOS30°=400
N；3
（2）对物体B受力分析，受重力、拉力、支持力和静摩擦力，如图所示：
根据平衡条件，有：
N+Tsin30°=GB
TCOS30°=f
求得：
N=800N
f=200
N3
答：（1）OC绳的拉力为400
N；3
（2）物体B所受地面的摩擦力为200
N，支持力为800N．3
People use their mouths for many things. They eat, talk, shout, and smile. So there are many expressions about mouth. For example, if you say things about a person, the person will be unhappy and say “Do not bad mouth me.” Sometimes, people say something to a friend or family member that later they regret they hurt that person’s feelings. The speaker might say: “ I really put my foot in my mouth this time.” If this happens, the speaker might feel down in the mouth. Information is spread through the mouth. This is general communication people, like friends talking to each other. “How did you hear about that movie?” Someone might ask. The may be : “Oh, by word of mouth.” A more official way of getting information is through a company or government mouthpiece. Sometimes, a person has a bad or unpleasant with another person. He might say: “It left a bad taste in my mouth.” Or a person might have a very experience, like being chased by an angry dog. He might say: “I had my in my mouth.” Some people have lots of money because they were into a very rich family. You might say such a person “was born with a silver spoon in his mouth. ” And this rich person is the opposite of a person who lives from hand to mouth.
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