题干

小明用如图所示的装置探究杠杆的机械效率,O为支点.他先将钩码悬挂在b点,在a点竖直向上匀速拉动弹簧测力计,拉力为F1,a点上升的高度为h.然后他将钩码移至c点,仍在a点竖直向上匀速拉动弹簧测力计,拉力为F2,a点上升的高度仍为h.两次拉动杠杆所做的总功分别是W总1、W总2,有用功分别是W有1、W有2,机械效率分别是η1、η2,以下分析正确的是(   )

A:F1>F2

B:W总1>W总2

C:W有1=W有2

D:η1<η2

上一题 下一题 0.0难度 选择题 更新时间:2018-11-01 11:15:33

答案(点此获取答案解析)

D

同类题5

完形填空

     People have been raising pigeons(鸽子) for about 10,000 years. And pigeon  keepers use  different kinds of pigeons for different 1Some people use pigeons for 2 For example, in parts of China people cook a special meal with pigeon to celebrate the New Year.

Some people also keep pigeons because they look especially 3There are many interesting kinds of these “pretty” pigeons. Different kinds of pigeons have interesting features 4 long, soft tail feathers or designs on their heads. The owners of these kinds of pigeons enter them in 5  and the best looking pigeons are chosen to win prizes.

     But pigeons have many othe6   too. Pigeons can fly long distances without growing 7 . They also have another8sense. No matter how far from home they are, pigeons can usually fly back to where they9This is often called a “homing sense”. This sense makes them good for carrying10

     In many cities around the world, pigeons live in special houses, and pigeon keepers spend a lot of money and time11 their birds. When someone has a very good pigeon, he is very12    He may also have a way to make money.  But this is not usually the main13 people keep pigeons. For some people, they keep pigeons as a hobby.

     People love pigeons so much that they even take them14 when they move to a new country. They15 these birds as their children.  “When you are looking at a pigeon in the sky you feel free. It is the best feeling you have in a strange country”, a pigeon lover says.