🪢 向心力：把物体拉向圆心的那只手🪢 Centripetal Force: The Hand That Pulls Toward the Center
上一片里，转圈的物体一直在加速——加速度指向圆心。有加速度就有力。小火箭问：这只把物体拉向圆心的“手”，到底是谁？Last leaf, a turning object was always accelerating — toward the center. Acceleration means force. So the little rocket asks: who is the “hand” that pulls things toward the center?
第 1 步：绳子就是那只手Step 1: The string is that hand
小火箭做了个实验：绳拴小球转圈，谁在拉着球拐弯？The rocket runs an experiment: a ball on a string — who keeps pulling it around?
绳子绷得笔直，把小球一直往圆心拽。这个指向圆心的合力就叫向心力。它要多大？F = mv² ÷ r：转得越快，需要的力越大；转速不变时，把绳加长，需要的力也更大。绳一松，这只手就没了。The taut string keeps tugging the ball toward the center. That toward-center resultant force is the centripetal force. How big? F = mv² ÷ r: the faster the spin, the larger the force; and at the same spin rate, a longer string also needs a larger force. Loosen the string and the hand is gone.
可汽车转弯时并没有绳子——谁来当这只手？But a car turning a corner has no string — who is the hand there?
第 2 步：它不是新的力，是“分工”Step 2: Not a new force — a job title
向心力不是一种新性质的力，而是按方向起的“岗位名”：谁在拉物体指向圆心，谁就干这个活。汽车转弯，靠轮胎与地面的摩擦力；绳拴小球，靠绳的拉力；卫星绕地球，靠地球的引力。分工各不相同，方向都指向圆心。Centripetal force is not a new kind of force — it is a job title named after its direction: whoever pulls the object toward the center does the job. A cornering car relies on friction between tyres and road; the ball relies on the string's pull; a satellite relies on Earth's gravity. Different workers, all pulling toward the center.
如果这只“手”突然消失，会发生什么？What happens if the hand suddenly vanishes?
第 3 步：手一松，它沿切线飞出去Step 3: Let go, and it flies off along the tangent
绳子一断，向心力为零，小球不再拐弯，只能按惯性沿切线直线飞出——注意，不是沿半径向外飞。甩干桶就是这个原理：水滴拐不了弯，从桶壁小孔沿切线飞走，衣服就甩干了。Cut the string and the centripetal force is zero; with nothing to make it turn, the ball keeps its direction of motion (inertia) and flies off along the tangent — not outward along the radius. That is exactly how a spin dryer works: the droplets cannot turn, so they fly out through the holes along the tangent, leaving the clothes drier.
🎮 你来抡一抡（30 秒）🎮 Your turn to swing (30 seconds)
道理讲完了。拖绳长和转速滑块，看所需向心力怎么变；点“剪绳”，看小球往哪儿飞。Theory done. Drag the length and spin sliders and watch the required force; hit “Cut the string” and see where the ball flies.
一句话记住它：向心力不是新的力，是“分工”出来的、指向圆心的那只手；它一松开，物体就沿切线飞走。Remember it in one line: centripetal force is not a new kind of force — it is the toward-center hand on duty; the moment it lets go, the object flies off along the tangent.
向心力 = 指向圆心的合力，由实际的力充当：绳拉力、摩擦、引力……Centripetal force = the toward-center resultant, provided by real forces: string pull, friction, gravity…F = mv² ÷ r：转得越快越费力；转速不变时，绳越长也越费力F = mv² ÷ r: faster spin needs more force; at the same spin rate, a longer string needs more too向心力消失，物体沿切线飞出（惯性），比如甩干桶里的水滴With no centripetal force, objects fly off along the tangent (inertia) — like droplets in a spin dryer
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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