题干

两个可视为质点的小球ab,用质量可忽略的刚性细杆相连放置在一个光滑的半球面内,如图所示。已知小球ab的质量之比为,细杆长度是球面半径的倍。两球处于平衡状态时,细杆与水平面的夹角θ是:    (  )

A:45°   

B:30°   

C:22.5°   

D:15°

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D

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   Most schools are trying hard to improve their lunch programs and they want to offer the best food they can.
   Even if your school provides healthy options(选择),you may still want to have something delicious outside.How do you take control? Take a packed lunch to school!
   Here are the four reasons to pack your lunch and snacks at least twice a week.Control  Have you ever waited in the lunch line only to find when you get to the front that you don’t like what they’re serving?
   Variety  If you’re eating the same food all the time , your body probably feels ready for a change. Packing lunch a couple of times a week means you can enjoy some favorites that you might not find at every school.
   Money Serving Packing healthy snacks so that you needn’t go out for a fast-food lunch, chocolate or a soda! If so, you will save much money. And you can buy something you need.
   Warm Feeling   Remember when your mom or dad used to pack your lunch? Pack a lunch for yourself, maybe that is your favorite healthy food.
   Whether you pack or eat in the cafeteria(自助食堂),what’s important is that you make right choices. If you are worried that cafeteria doesn’t offer enough healthy choices, try to make changes. Ask a teacher or someone in food service for advice on hoe to get started.
Take a packed lunch to school

 
____  for packing your lunch and snacks twice a week
To control dietsIt helps ____ you eating the unhealthy    food and you needn’t wait in line.
To be ____ Your body needs____ _and you can enjoy some favorites which are not_____ at school
To____ moneyHealthy snacks from home are ____ ____ __than a fast-food lunch.
To feel warmIn the past, your parents packed lunch for you while you just do it____ _now.
ConclusionIt’s important to ____the right food. Go for advice on how to start.