# Newton law of universal gravitation

1. Jan 21, 2014

### Tiven white

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

Having problems on the method of calculating the acceleration due to gravity of the planet.

A certain planet has a diameter of 1715 km and a density of 5254 kg/m³. The planet has a moon that orbits every 4.46 earth-days.
What is the acceleration due to gravity associated with this planet? Answer in units of miles/hour

2. Relevant equations

V = (4/3)*∏*(r)^3

F = (G *M1*m2)/(r^2)
G - gravitational constant

F = (m)*(v^2)/(r)
V--velocity

3. The attempt at a solution

so after utilizing half the diameter given as the radius the formula for volume was used to calculate the volume of this planet. since density = mass/volume the equation was rearranged to find the mass of the planet. i am a bit stuck here since i find it difficult using the information for the orbit of the planet into this problem. any insights towards the solution for the acceleration would be appreciated.

2. Jan 21, 2014

### haruspex

OK, but not, in general, the same radius as in the previous formula, right?

It isn't entirely clear, but I assume you're being asked for the acceleration due to the planet's gravity at the moon's distance.
Let the distance from the moon to the centre of the planet be d.
In terms of that, what would the gravity at that distance be? What period of orbit would it give you?

3. Jan 22, 2014

### tiny-tim

Hi Tiven white!
you don't know v but you do know ω (the angular velocity), so use the alternative formula for centripetal force:
F = mω2r

(and that gives you two formulas for F, and there's only one value of r for which they're equal)

4. Jan 22, 2014

### SteamKing

Staff Emeritus
Acceleration due to gravity will not have units of miles per hour. Those are the units of velocity.