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answers:
Gray Bold say: Newton's law of universal gravitation states that a particle attracts every other particle in the universe using a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. In modern language, the law states: Every point mass attracts every single other point mass by a force pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them.
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Morningfox say: There is this little thing called gravity.
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Tom S say: Inertia mostly, and gravity.
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injanier say: Speed doesn't cause inertial forces, acceleration (change in speed or direction) does. While there is some centrifugal force involved, it is very small, because the rotation rates are so slow.
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quantumclaustrophobe say: Earth's gravity is strong enough (at our distance from it's center) to keep us firmly on the surface.
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Gary B say: gravity
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Good Man say: Due to gravity of earth
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Bill say: Because of a thing called gravity, the same reason the Earth doesn't fly away from the sun.
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Raymond say: If you sit on a train, and the train moves, why don't you fly off towards the back of the train?
You are sharing the speed of the train. Therefore you are stable relative to the train.
On Earth, you are sharing Earth's speed around the Sun. Therefore, you are stable relative to Earth. Or, if you want it in the weird (but still true) version relative to the Sun: you are flying off at exactly the same rate as the Earth is flying off.
Because it is orbiting at 30 km/s (average), the Earth is weightless RELATIVE TO THE SUN.