题目:跨过两只定滑轮的绳子的中点处栓住一个重物,重物质量为M(图1)。今以相同的速度v向下匀速拉引绳端使重物竖直上升,重物的速度逐渐变_,图1中θ变为_时,重物的速度为2v,此时,绳的拉力为___。答案与提示是:重物速度v’=v/sinθ,所以当重物上升时,重物的速度逐渐变大,由v’=v/sinθ可知,当θ=30°时v’=2v。此时重物所受三力互成 Title: A heavy object is bolted to the midpoint of the rope across the two fixed pulleys and the weight is M (Figure 1). Now we pull the rope end at the same speed v down to make the weight rise vertically, and the weight speed changes gradually. In Figure 1, when θ becomes _, the weight’s speed is 2v, at this time, the rope’s tension for___. The answers and cues are: the weight velocity v’=v/sin θ, so when the weight rises, the velocity of the weight gradually increases, known as v’=v/sinθ, and when θ=30°, v’=2v. At this point, the heavy objects are subjected to three forces