Physics Question

[madoscar65]

Hi peoples,
I've been trying to do this question for ages, but I can't solve it... I've tried using ratios but did not get the right answer. Here's the question : A spacecraft leaves Earth to travel to the Moon. How far from the centre of the Earth is the spacecraft when it experiences a net force of zero?

Answer is 3.08 x 10^7m

Thanks

[Whatlol]

What values can you use for the question

[madoscar65]

No values are given... this question is very weird..

[Whatlol]

This isnt from the heineman text book? i recall doing a question like this last year.

But this is how i would go about approaching this question

Fg(earth on spacecraft) = GMem /(Re)^2 , where Re is the distance from the centre of the earth to the astronaut.
Fg(moon on spacecraft) = GMom/(R-Re)^2 , where R is the radius of the moons orbit around earth

Since the forces are equal at this point, Make the two equations equal to each other.

[madoscar65]

Yeh that's what I did first, but I didn't get close to the answer.. Unless throughout the process, I've done something wrong

[Whatlol]

Well you cant get an answer unless you use the values for the mass of earth and the moon, also the radius of the moons orbit.
 
infact i found a question in the heineman text book which is essentially exactly the same as the one you put up,  3.2 question 12.

the answer, interestingly is 3.4x10^8m, which is the answer you get by using the approach i put up earlier... so i dunno whats going on