某人将一根木棒的一端抬起，另上端搁在地上．在抬起的过程中（棒竖直时除外），所用的力始终竖直向上，则用力的大小（ ）
A．保持不变
B．逐渐增大
C．逐渐减小
D．先减小后增大
若力总是竖直向上，因为△OAC∽△OBD，所以
=OA OB
为定值，即力F的力臂和重力力臂的比值为定值，根据F×OB=G×OA可得，F与G的比值为定值，故力F保持不变．OC OD
故选A．
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to children, but also good for all ages. The following are chosen 2_____ his letter. 1. Life is short. While you are 3_____ it today, you'll realize you are at the end of it tomorrow. So the earlier you start to value your life, the 4_____ you can enjoy it. 2. You might not be successful 5_____ you don't study hard, although a lot of successful people haven't 6_____ higher education. 3. I don't expect you to support (供养) me for the rest of 7_____ life, so I'm not going to do the same for you. You will be living on your own when you grow up. 4. You can require yourself to be nice to others, 8_____ you shouldn't expect the same from others. 5. I've been buying the lottery ( * * ) for almost twenty 9_____, but I'm still pool. I have never got the 10_____ place even once. So you have to work hard to be successful. There is no free lunch in the world. |