If the gravity of the moon is the equvalant of a pin head held out as far as u can reach, the how is

Question

The moons gravity is the equivilant of a pin head held out above your head, then how is it able to pull back the seas causing the tides?

Answer 1

The tides aren't caused directly by the moon's force of gravity. They are caused by that force's gradient over the entire ocean, which is enormous.

Answer 2

Gravitation is a reciprocal interaction between bodies possessing mass.

You have far less mass than an ocean, hence you and the Moon interact less gravitationally than the Moon and an ocean.

Answer 3

This is an excellent question!

But first, the analogy is not quite correct.  I did some quick calculations and found that the strength of the moon's gravity (as felt from earth) is equivalent to the following:

535 tons held at arm's length above your head; or:

A pinhead held about 1/700 inch above your head.

Still, even with the corrected numbers, it's a valid question.  If you stand in the basement of a skyscraper, the gravitational pull of the skyscraper probably exceeds that of the moon.  So why doesn't the skyscraper produce tides in (say) a sink full of water?

The answer, as José points out, is that the tides are not caused directly by the strength of the pull, but also by how the pull changes with distance.  This uses a different formula than the calculation of raw strength, and depends strongly on the distance from the pulling body.  When you take that into account, it turns out that a very massive, faraway object has a much greater tidal effect than a tiny, close object with the same gravitational pull.

Answer 4

If?  Gravity is a force.  A pin head (whatever that is (I'm not dismissing the possibility that this question was asked by one)).  Is an object.

They are not equivalent in a vast vast number of ways.

F = Gm1m2/r² (Newtonian)  So, the force between two bodies is proportional to the mass of each .

AND (this is important) it is inversely propotional to the distance (r) between them.  So lets say MY head weights 10 pounds (everybody says I'm really dense).  And lets say a pin weighs 0.0001 pounds.

And lest say that I hold the pin 1 foot above my head.

So lets figure what that force would be

F=G * 10*0.0001/1² = 0.001G  (where G is in applicable units)

for F between me and the moon m2=1.6E23 pounds and r=1.3E9 feet

F= 1000000G.

Obviously, you grossly misunderstood whatever you heard.

Also obviously.  At a far enough distance the gravity of the Moon will exert the same force on you as a pin held at arms length does.  Just not on this planet. (have to go about 10,000 times further away from the moon as we are now.)

Answer 5

Where did you get that example?????? HUH???

Gravity exerted by moon or by a pinhead does not depend on its mass alone. The mass of other body upon whom it is exerting force also matters.

So ratio of pinhead to you is not equal to that of moon and water on earth.

The force is much greater...

Answer 6

uh.. the moon's gravity is 1/6th that of earth's.... i'm not sure how a pinhead being held from your head could replicate this....  

Answer 7

I question that the Moon's gravity at Earth is as small as you say (same as pinhead 3 feet away). I haven't done the math, but it sounds wrong.