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Thread: Gravity

  1. #1 Gravity 
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    The position of the moon above the earth causes the tides due to the moons gravity, so does this mean the moons gravity pull also slightly affects the weight of items on earth in time with the tides?


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by cluelessone View Post
    The position of the moon above the earth causes the tides due to the moons gravity, so does this mean the moons gravity pull also slightly affects the weight of items on earth in time with the tides?
    Yes it does. The effect is really tiny - someone on another forum recently calculated the effect as being about 1 part in 10 million (1). You will weigh his much less when the moon is directly overhead or directly beneath your feet. You will weigh correspondingly more when it is at the horizon.

    (1) I didn't check this before but it should be easy to do using Google, search for: (mass of moon / mass of earth)*(radius of earth / (356400 km + 406700 km)/2)^2
    This gives a result of: 2.15158153 10-7 (that is how much smaller the moon's gravity is than the earth's; 2 parts in 10million)


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    Comet Dust Collector Moderator
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    The sun also has a similar but smaller effect, so you weigh less at noon and midnight.
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    The earth is affected by the gravity of the moon as the Earth-Moon system is a system and by that nature share a centre of interacting gravity which we call a 'barycentre'. While the effects of objects being pulled towards the moon is only marginal and fractional by comparison to earths gravity. It is still strong enough to make the earth orbit a point 1000km on average below the earths surface at the point closest to the moon, but no-where near strong enough to move objects on earth or alter weight in any discernable way:

    celestia celestia4all EARTH-MOON BARYCENTER - YouTube
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