physics gravitational fields question - OCR A physics - forces fields and energy!

A spacecraft on a journey from the earth to the moon feels no resultant gravitational pull from the earth and the moon when it has travelled to a point of distance between their centres. Calculate the masson of the moon, using the value for the mass of the earth as 6x10^24 kg.

Please help!

Reply 1

F1 fanatic

16

cpat

A spacecraft on a journey from the earth to the moon feels no resultant gravitational pull from the earth and the moon when it has travelled to a point of distance between their centres. Calculate the masson of the moon, using the value for the mass of the earth as 6x10^24 kg.

Please help!

It's just a case of balancing the gravitational forces for the Earth and the moon. The resultant force is zero so:

\frac{GM_{moon}m}{(R_{EM}-x)^2}=\frac{GM_{Earth}m}{x^2}

you need more information though, are you given the distance from the Earth to the moon, which I called

R_{EM}

and the distance from the earth to the rocket, x?

Reply 2

aqfrenzy

2

Ah I did this question the other day. The question states it's 0.9 the distance between Earth and Moon (starting at Earth).

So R for Earth is 0.9R and for Moon is 0.1R. Then you just equate as mentioned above. You can take g as being equal so it's just the GM/R^2 which gets equated for Earth and Moon.

Reply 3

F1 fanatic

16

aqfrenzy

Ah I did this question the other day. The question states it's 0.9 the distance between Earth and Moon (starting at Earth).

So R for Earth is 0.9R and for Moon is 0.1R. Then you just equate as mentioned above. You can take g as being equal so it's just the GM/R^2 which gets equated for Earth and Moon.

g as being equal? That's not correct, and you don't need to assume it. It is g=GM/R^2 where the Ms are different, but you don't need to assume it anyway since you know the mass of the earth and the radius R will cancel out. The mass of the rocket also cancels out.

physics gravitational fields question - OCR A physics - forces fields and energy!

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