# Homework Help: Consider a planet in some solar system which has a mass double

1. Nov 9, 2011

### draotic

1. The problem statement, all variables and given/known data
Consider a planet in some solar system which has a mass double the mass of earth and same density as of earth . What is the weight of object on the planet in terms of 'W' , where 'W' is weight of object on Earth

2. Relevant equations
W=mg ... density = M / V . .... g=GM/R2

3. The attempt at a solution
since they both have same density..
i equated their M/V ratios and it gets me the ratio of their radii ..
but it still gets me nowhere

2. Nov 9, 2011

### Pengwuino

Re: Gravitation

Letting 'D' indicate the information you know for the double sized planet, you know

$M_{D} = 2M_{Earth}$

and you know $R_{D} = X R_{Earth}$ where X is whatever you determined the ratio between the radii are. So simply find what the gravitational acceleration on this new planet will be with $g = {{GM}\over{R^2}}$. The trick will be being able to write 'g' as $g = Y \times {{GM_{Earth}}\over{R_{Earth}^2}}$ where you have some constant, 'Y', multiplying the original values known for the Earth. Those known values you know gives $g_{Earth} = 9.8 m/s$.

You'll have some multiplicative value in front that will tell you how many times stronger or weaker the gravity is.

3. Nov 9, 2011

### draotic

Re: Gravitation

thanks , got it

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