Gravitational acceleration from moon

1. Jan 1, 2012

Irishwolf

Hi ,
How could I calculate the acceleration of an object on the earths surface , say at the equator , from the influence of the moon , when the moon is directly overhead?

I have calculated the force of attraction from the moon to be 1.99 x 10^(20) N.

Now to find the gravitaional acceleration do I use ==> a=F/M
Where big M = mass earth?
I do I use the mass (m) of the moon?
Or is this not the way to calculate it?
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Another attempt of solving this was i used the formula for influence of moon on tides:
a= GMm/(R-r)^(2) - GMm/(R^(2))
and grav acc due to moon is 0.000001131 m/s^(2)
But someone said I should solve it with the above equation?

2. Jan 1, 2012

Neandethal00

I'm not sure I understand your question. If your question is what is the net acceleration of an object on earth considering the gravitational effect of the moon, you may look at it with this formula:

a = GMearth/R2 - GMmoon/D2

Mearth = mass of earth
Mmoon = mass of moon
D = distance from center of moon to the object.

3. Jan 1, 2012

Irishwolf

Hi yes that was my question thanks,
But why is that a different formula than the one used to calculate the moons influence on tides?

4. Jan 1, 2012

Neandethal00

Not sure about tide formula. What does that do?
Note, mass of the object is irrelevant. For tide, is there a
well defined mass? You can treat all ocean water as one massive object,
but then acceleration of what water? Our simple formula then may not be
very accurate.