question on gravitation?

Question

)Calculate the gravitational potential of a point on the surface of the moon closest to the earth( ans : -3.9X10^6 J/kg)
Data : Mass of Earth : 6 X 10^24m
Mass of moon : 7.4 X 10^22m
radius of moon : 1.7 X 10^6 m
dist from centre of earth to centre of moon : 3.8 X 10^8m

what does it mean by a point on the surface of the moon closest to the earth?

if possible, do include a short statement of what the calculations involve. thanks alot!

Answer 1

The distance from the Earth to the _center_ of the Moon is different than the distance to something on the _surface_. Something on this side of the Moon is 1.7x10^6 closer and something on the other side of the Moon is 1.7x10^6 farther.

You must also consider that the gravitation from both the Earth and Moon affect the potential.

Answer 2

Since the moon has a definite diameter, the point on the surface of the moon closest to the earth will be nearer to the earth by the radius of the moon. Pl. work the problem keeping that in mind.

Answer 3

this question is based on newtons law of universal gravitation, ie the gravitational pull between two objects.
the equation is:
Fgrav = G*Mass1*Mass2/Distance^2
where G is the constant 6.67x10-11
the distance is from the center of the first mass to the center of the second mass

for this problem it wants the gravitational pull from an object on the surface of the moon, subtract the radius from the distance between both centers
3.8x10^8-1.7x10^6=378,300,000
the final equation is:
F=(6.67x10^-11*7.4x10^22*6x10^24)/(378,300,000)^2
the answer i get is:
2.069x10^20 in meters which is different from the answer you provided