27-09-09 Moon and a stone...

Suppose there was a planet of mass M which was free from any other force from a planet or star...
and this planet had a moon of mass m.

Now, the rotation is such that the same surface of the moon is visible from the planet...
The radius of the planet is R, the moon is r

Find the minimum separation such that any lose stone on the moon is pulled by the planet!!

10 Answers

62
Lokesh Verma ·

No one!

This one is simple!

11
virang1 Jhaveri ·

Any stone will fall to the planet only if acceleration due to planet is more than moon
Therefore
a due to moon = Gm/r2

And
A due to planet =GM/x2

GM/x2>Gm/r2
M/m>x2/r2
x<r√(M/m)

x = D + R
D<r√(M/m) - R

62
Lokesh Verma ·

you have missed out something!

11
virang1 Jhaveri ·

i cannot think of anything elese

3
msp ·

field due to moon on its surface is GMmoon/r2=E1

field due to planet on moon's surface is GMplanet/(D-r)2=E2

E1<E2

we will get D=Distance between the com of moon and earth.

62
Lokesh Verma ·

no sankara...

Hint: You cannot always apply ΣF =0

That is what you all are doing.. net force is zero... (Which is not true here!) why?

1
Arshad ~Died~ ·

net force cant be 0 here because both the planet and the moon are accelerating with respect to each other............am i right?

62
Lokesh Verma ·

yup arshad..

so with that in mind.. can you finish this one?

3
iitimcomin ·

sorry i didnt have patience to read the above disc!!....sorry if it has gone terribly rong!

GMm(stone)/[d-r-R]2 = Gmm(stone)/r2 + m(stone)W^2(d-r).....(1)

also

GMm/d2 = mw2d ......(2)

eleminate w frm both equations and get the disired 'd'.......

3
msp ·

@iit can u explain ur 2nd eqn.

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