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原题呈现:21.(17全国Ⅰ)如图1所示,柔软轻绳ON的一端O固定,其中间某点M拴一重物,用手拉住绳的另一端N。初始时,OM竖直且MN被拉直,OM与MN之间的夹角为α(α>π/2)。现将重物向右上方缓慢拉起,并保持夹角α不变。在OM由竖直被拉到水平的过程中AD A.MN上的张力逐渐增大B.MN上的张力先增大后减小C.OM上的张力逐渐增大D.OM上的张力先增大后减小
The original title shows: 21. (17 nationwide) As shown in Figure 1, one end of the soft light rope ON fixed O, the middle point of a M Mianshe a heavy object, hand pull the other end of the rope N. Initially, OM is vertical and MN is straightened. The angle between OM and MN is α (α> π / 2). Now heavy objects slowly pulled up to the top right, and keep the included angle α unchanged. In OM by the vertical process is pulled to the level of AD A.MN tension gradually increased B.MN tension on the first increase and then decrease C.OM on the tension gradually increased D.OM on the tension Increase and decrease