The Earth’s gravity is strong enough to keep The Moon, an immense celestial body, from flying off into space. It is also weak enough to allow us to jump and throw things in the air. How does this work?

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Similarly, why does The Moon’s gravity affect the tides but won’t, for example, cause a paperclip to slide across a desk?

In: Physics

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Anonymous 0 Comments

Basically, because the gravity you need to keep hold of the moon or to create the tide is *tiny*.

Being in orbit is just falling all the time, but moving fast enough to miss the ground by the time you get there. The force of gravity between earth and the moon is so small that it takes about half a month for it to reverse direction. That would be like jumping so high that you don’t come down for 2 weeks.

Obviously we can’t do that here, but gravity gets weaker very fast, as you move away from earth. Out where the moon is you wouldn’t even feel it, I don’t think.

The tide, meanwhile, doesn’t take much because it doesn’t actually move very far. It might seem big to a human on a beach, but the few meters between high and low tide is only a tiny fraction of the depth of the ocean.

It doesn’t take much strength to shift something such a small amount.

Also, the reason a seemingly small amount of gravity can have big effects on big things is that it’s working on every piece of whatever it’s moving.

You might not feel the pull of the moon on a human scale, just like you can’t see the tide in a human-sized pool of water, but it’s there.

When the moon pulls on the ocean, it’s pulling on each human-sized bit of it separately. You still couldn’t see the effect looking at any one of them, but add them all up and you get the kind of forces you need to shift whole oceans a *really tiny* amount. It’s like watching the edge of a puddle with a really good microscope. There’s a tide there, or in a cup of water, it’s just too small to see.

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