# Universal gravitational problem

1. Jan 8, 2012

### anna sung

1. The problem statement, all variables and given/known data

Planet X has a mass 8 times that of Earth and the acceleration due to gravity at its surface is 19.6 m/s2. The radius of the planet x compared to earth is:

2. Relevant equations

Fg = gm1m2 / r^2

3. The attempt at a solution

Is earths gravity 9.8 and the gravity of planet x is 19.6.
Where would I put these values in ?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 8, 2012

### Staff: Mentor

Check your relevant equation what do each of the variables and constants mean?

In particular, f=Gmm/r^2 where G is what?

3. Jan 8, 2012

### anna sung

Isn't g universal gravitational constant which is 6.67 x 10^-11?
Or is this where the 19.6m/s^2 should go into?

4. Jan 8, 2012

### wetlife

G*m_1 is the acceleration due to gravity for a body

5. Jan 8, 2012

### ehild

Denote the universal gravitational constant by G. People use the notation g for the gravitational acceleration on the surface of the Earth: g≈9.81 m/s2

According to the law of universal gravitation, the force of gravity on an object of mass m is F=GmM/R2 at the surface of the Earth. M is the mass of Earth (5.97 x 10^24 kg) and R is the Radius of Earth (6.37 x 10^6 m).
The gravitational acceleration is F/m=g=GM/R2. Plug in the data: it is 9.81 m/s2

19.6 m/s2 is the gravitational acceleration at the surface on an other planet. How does it related to the radius and mass of the planet?

ehild

6. Jan 9, 2012

### anna sung

So f=mg=GmM/r^2
Then say the mass of earth is 2. Planet x is 8 times of the earth, so then 16.
Earth has gravity 9.8 and planet x has gravity 19.6.
Then the equation is really this: g=m/r^2
So earth is square root ( 2/9.8)=0.45
planet x is square root (16/19.6) = 0.903
Oh, okay yeah and it's twice larger.
Okay I get it,
Thank you so much.^_^

7. Jan 10, 2012

### ehild

You are welcome

ehild