$\frac{2}{3}$ ∶4＝x∶ $\frac{1}{2}$

x＝____ 

实验步骤如下：

（一）用调好的弹簧测力计测出1个钩码，所受的重力G．如图所示组装实验器材，用弹簧测力计竖直向上匀速拉绳子自由端，绳子自由端所受拉力用F表示，绳子自由端移动的距离用s表示，钩码上升的高度用h表示。将G的数值记录在表格中，用弹簧测力计测出F，用刻度尺分别测出s、h，并把测量数据记录在表格中；

（二）保持动滑轮所挂钩码不变，依次改变绳子自由端移动的距离s，仿照步骤（1）再做5次实验，并把各次的测量数据记录在表格中；

（三）利用公式η= $\frac{W_{有用}}{W_{总}}$ = $\frac{Gh}{Fs}$ ，计算各次动滑轮的机械效率η，并将η值记录在表格中。

A: Hello, Jim! What are you doing now?

B: Oh, I'm reading my report card.

A: I suppose you have passed all the exams. You are always hard-working.

B: It's true that I always do my homework by myself. But I often get nervous when I see the envelope from school.

A: How about your math this time? I know you don't do well in it.

B: It was lucky that I passed the math exam this time. We had a new math teacher this semester. She is a university graduate and works as a volunteer in our school. Her teaching has a good influence on my study and starts to open my eyes to the outside world.

A: That's really great. Does she like living in our hometown where the air is thin?

B: She said she felt happy with her decision.

A: By the way, I got a message for you. Your mother asks you to look after yourself well.

B: OK. Thank you.