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a sag in [sth]
There is a sag in the wire.
电线有一处下垂。
下垂；下陷
先理解核心含义，再看它在真实句子里的用法。
下垂；下陷
The state of sinking or bending; a droop.
There is a sag in the wire.
电线有一处下垂。
下沉或弯曲的状态；下垂。
The state of sinking or bending; a droop.
导线、电缆、链条或绳索悬挂在相邻两点之间的高度差。
The difference in elevation of a wire, cable, chain or rope suspended between two consecutive points.
曲面顶点与边缘之间的高度或深度差,特用于镜子或透镜等光学元件。
The difference in height or depth between the vertex and the rim of a curved surface, specifically used for optical elements such as a mirror or lens.
表面(如座椅、地面等)下沉或下垂之处,类似凹陷或山脊上的低洼。
A place where the surface (of a seat, the earth, etc) sinks or droops, like a depression or a dip in a ridge.
There is a noticeable sag in the middle of the mattress.
床垫中间有明显的下垂。
The bridge developed a slight sag after the heavy trucks crossed.
重型卡车经过后，桥面出现了轻微的下垂。
The shelf has a sag from too many books.
架子因书太多而中间下沉。
There is a sag in the wire.
电线有一处下垂。
The sag of the shelf is noticeable.
架子的下垂很明显。
The fabric has sags in several places.
布料有多处下垂。
Not this
There is too much sag on this shelf.
Use this
There are too many sags on this shelf.
学习者将sag视为不可数名词，但sag是可数名词，复数形式为sags。汉语中“下垂”常被视为不可数，导致错误。
Not this
The mattress has a drop in the middle.
Use this
The mattress has a sag in the middle.
学习者混淆了drop和sag，汉语中“下垂”有时被泛译为drop，但描述物体下凹时应用sag。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
电缆垂度
The design must allow for cable sag under heavy loads.
设计时必须考虑重载下电缆的垂度。
支撑点之间的下垂
The sag between supports increased after the storm.
暴风雨后，支撑点之间的下垂增大了。
中间下陷
There is a noticeable sag in the middle of the old bed.
旧床中间有明显下陷。
肩膀下垂
The sag of his shoulders told us he was exhausted.
他肩膀下垂，说明他精疲力竭。
绳线的垂度
Measure the sag of the line before tightening it.
收紧绳子之前先测量一下绳线的垂度。
Use sag naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
把sag想成‘撒’下去：重物压着，中间‘撒’出弯垂（sag），如绳子、沙发下垂。
语域：mixed 常见场景：日常描述、工程/物理、光学设计 作名词时，通常指下垂或弯曲的状态；在工程和光学领域多用于表示导线、镜面等的垂度或弛度。整体语气偏中性，但具体专业场合会更正式。
All listed words end in the /æɡ/ sound and rhyme with sag. Beware lookalikes: 'sage' /seɪdʒ/ and 'saga' /ˈsɑːɡə/ look similar but do not rhyme.
Dictionary facts from ECDICT, WIKTIONARY.