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Let’s look first in the vertical direction: We have a downward pull from gravity, and the strength of that force depends on the mass of the book (m) and the gravitational field (g) of the planet you’re on (Fg = mg). But the book isn’t accelerating downward, so there must be an equal force from the table pushing up. We call this a “normal force.” Result: The net vertical force is zero.
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2025年让我感到惊喜的变化是“新老人”群体的崛起。这一代60后群体拥有更强的消费能力和自主意识，正推动市场从“生存型养老”向“品质型养老”跨越。他们不再满足于简单的照料，而是追求精神满足与生活品质，倒逼市场涌现出老年旅游、在线教育等新业态。
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