题干

如图所示,甲、乙两船的总质量(包括船、人和货物)分别为10m、12m,两船沿同一直线、同一方向运动,速度分别为2v0、v0.为避免两船相撞,乙船上的人将一质量为m的货物沿水平方向抛向甲船,甲船上的人将货物接住,求抛出货物的最小速度.(不计水的阻力)

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答案(点此获取答案解析)

解:设乙船上的人抛出货物的最小速度大小为vmin,抛出货物后船的速度为v1,甲船上的人接到货物后船的速度为v2

取向右为正方向,由动量守恒定律得

乙船与货物:12mv0=11mv1﹣mvmin

甲船与货物:10m2v0﹣mvmin=11mv2

为避

同类题4

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    Hannah Levine decided she wanted to give hugs to all of the children and families in need at local hospitals.

    Because she couldn't give them one by one, Levine, then a sixth-grader, decided she would use her talents(才能) to do the next best thing. She began to knit(编织) hats, scarves, and blankets for families staying at the Ronald McDonald House at Stanford. Her creations also went to Bundle of Joy, a program that provides newborn baby items for families in need, and to Knitting Pals by the Bay, a local organization that provides hand-knitted caps to cancer patients.

    “I love to knit, and I thought it would be a great idea to make all these handmade items for kids and adults who need them. It would be like a hug for them,” Levine explained.

    Levine started the project about a year ago. “I think it's just really fun to do, and it keeps me busy,” said Levine, now 13.

    Once she got started, Levine realized that her project could be much bigger than the goods she was able to produce with just her own hands. So she sent emails to her school and communities(社区), asking for knitted donations(捐赠物) to the project she named “Hannah's Warm Hugs”. She also posted advertisements at Starbucks and other locations in her area. The warm goods began to gush in.

    “It was amazing; more strangers than people she knew started dropping donations at our door,” said Levine's mother, Laura Levine. “We ended up with this huge box of items she was donating.”

    The knitted items numbered in the hundreds. Levine made her first round of donations around Hanukkah (an eight-day Jewish holiday in November or December) and later received thank-you letters from the organizations. Levine is still knitting, and she said the project will continue.

    “It has turned into a bigger thing than she had thought,” her mom said. “It made her feel pretty good; it made us feel pretty good.”