建筑工人用如图所示的定滑轮装置运送建筑材料，质量为70.0kg的工人站在地面上，通过定滑轮将20.0kg的建筑材料以0.500m／s2的加速度拉升，忽略绳子和定滑轮的质量及定滑轮的摩擦，则工人对地面的压力大小为多少？（g取10m/s2）
解：对建筑材料进行受力分析，根据牛顿第二定律有F-mg=ma，得绳子的拉力大小等于F=210N
然后再对人受力分析，由平衡的知识得Mg=F+FN，得FN=490N
根据牛顿第三定律可知人对地面间的压力为490N
Teaching kids to fish sometimes puzzles us parents. Now I’d like to give you a few tips. First of all, start them young. Get them in a boat, or on a pier(码头), or on the bank at as young an age as you possibly can. Obviously this means being extremely conscious about their safety. Whatever you do, DON’T plan on any fishing yourself. If fishing is going to happen, the fishing will be done by the kids with your help! This lets you pay attention to them and not ignore them while you fish. Next, make sure you plan the first trips to someplace where you can actually catch a lot of fish. At this point, it doesn’t matter what kind or how big, just make sure they can catch some. A small child’s patience is very short, and waiting 30 minutes for a bite will turn them off in a hurry. You may need to go to a small pond where the fish are dying to eat bread balls. Any type of fish will do. Remember, this is for them, not for you. As they fish, cheer for every fish they catch. Make a really big deal out of it. Take pictures. And if it’s possible, take some home to clean. Let the child help you clean with a table knife, so he can’t get hurt. Make sure he makes and sees the connection from catching to cleaning to cooking. As soon as you have the fish cleaned, it’s into the kitchen to start cooking. Make him flour and cornmeal the fish. Let him see the fish cooking, and talk about how good they are. Then when it’s ready, make sure you brag（吹嘘） on the fact that he (or she ) caught these fish, and “aren’t they good?” In this way, I’m sure your kids will be interested in fishing and good at fishing. Title: Teach a kid to fish
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