Physics

[dmja]

Hi there,

My question is relating to an advanced level text book question.

 

'Stars a and b have the same density, and star a is 27 times more massive than star b. Calculate the ratio of the gravitational field strenght on star a to that on star b'

 

I hope you would be able to explain how you came to the answer, please.

[timo]

Providing you with the answer plus the way to get there when you haven't even said how you want to approach the question is not the idea of the homework help forum. So please say a few words how you think you might solve the question, what you already tried to solve it, and where you got stuck. One thing that I find remarkable though: Do you actually understand what the question asks you to calculate? I don't. Does the question ask for the gravitational force applied on a person standing on their surface or does it ask about the ratio of the forces the planets exert onto another? Hint: if you can answer that then set up a formula for the respective forces and you're almost done.

[dmja]

Thankyou for the reply.

I think that it is asking about the ratio of the forces the stars exerted onto one another. Using the following formula I know how to calculate the field strenght.

If the the star A is 27 times more massive (DOES THE WORD MASSIVE RELATE TO THE MASS inthe equation?)

 

(how do I know the distance between the stars- it doesn't say in the question)

 

The equation for density is mass/volume, but the stars are 27 times more massive so how does this relate the equation. Therefore the volumes must be different.

 

I don't know how to put this lack of information into the equation and then change it into a ratio.

 

 

 

 

 

[timo]

Thankyou for the reply.

I think that it is asking about the ratio of the forces the stars exerted onto one another. Using the following formula I know how to calculate the field strenght.

The formula is correct but note that the field strength g is not exactly equal to the force F exerted on an object in that field. The question asked you to compare the forces, not the field strengths. Btw.: It's cool that you take the effort to create an image of your formula, but you can also just write g = G*M/(r^2); people will also understand that.

 

If the the star A is 27 times more massive (DOES THE WORD MASSIVE RELATE TO THE MASS in the equation?)

Yes, that's the only meaning that seems to make sense, here.

 

(how do I know the distance between the stars- it doesn't say in the question)

Then keep it as the unknown variable "r" for now and hope that it drops out in the final answer.

 

 

The equation for density is mass/volume, but the stars are 27 times more massive so how does this relate the equation. Therefore the volumes must be different.

The star with 27 times the mass must have 27 times the volume of the other star, then. But since volume wasn't in your equation above, that information does not seem necessary at the moment. What you do know is that you have two masses m1 and m2, and that they are related by m1 = 27*m2 (assuming planet 1 is the heavier one), so each appearance of m1 in your equations can be replaced by 27*m2.

 

 

I don't know how to put this lack of information into the equation and then change it into a ratio.

Just keep parameters about which you know nothing as unknown variables (like the distance "r" between the two planets) and see how far you get. In this case, all of the unknowns drop out at some point (since homework questions tend to be give all the information required to solve it).

[swansont]

If you find that you need to work with the different volumes, you were told that they have equal density, which means you should be able to solve for the relative radii of the two bodies. But it doesn't appear to be asking for surface gravity, so finding the ratio of the field strength at an arbitrary distance would seem to be sufficient.