题干

如图所示,一个半径为r,质量为m的半球,放在容器内,半球的底面与容器的底部紧密接触,容器内装有密度为p的液体,液面高为H.已知球体的体积公式是V= π
4
r
3
3
,球表面积公式是Sm=4πr2圆面积公式是Sm=πr2则由于液体重力产生的对半球表面向下的压力为____

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答案(点此获取答案解析)

πr2ρgH﹣
2
3
ρgπr3

同类题3

阅读理解

D

    “Volunteering makes you look at the world differently. You see how little things can change a person's life for the better, which makes everyone a better person.” Mason, a volunteer said.

    National Volunteer Week started in 1974 and is held every April. It is a time to thank volunteers for their achievements. It is also a perfect opportunity (机会) to encourage others to take their first step toward becoming a volunteer. Making the decision to take the first step can be the biggest problem, because they often wonder if they will be able to meet the organization's expectations (期望). Some people fear not knowing anyone else in the group. Not having enough time also prevents some from sharing their abilities.

    The following tips can help if you have some of these worries.

    ★Start out slowly, don't add too much pressure. Even a few hours a month can make a big difference in someone's life.

    ★Choose an organization with the same interests and common values.

    ★Take part in a training meeting for new volunteers, even if it is not asked.

    ★Work with a veteran volunteer. He volunteered a lot, he can help you increase the confidence and completely understand the organization's expectations.

    ★Invite a friend or family member to serve. It is a good experience to volunteer with them.

    ★Finally, it is most important to enjoy the volunteer experience and to remember that not all volunteer experiences are perfect. If one experience doesn't work, don't give up and you'll surely find the right opportunity.

    Spend some time on volunteer work and you will see great changes in people's lives. As Mason discovered during his volunteer experience, “…little things can change a person's life.”