# ~Gravity on a distant planet

1. Homework Statement
You have been visiting a distant planet. Your measurements have determined that the planet's mass is twice that of earth but the free-fall acceleration at the surface is only one-fourth as large. Radius = 1.8*10^7meters
Mass = 1.19484 × 10^25 kilograms

2. Homework Equations
U_g = -(G*M*m)/r
v_escape = sqrt(2GM/r)

3. The Attempt at a Solution

I just plugged in the variables to the espace velocity equation and get 94km/s, but thats incorrect.

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The radius was given in the first part of the problem.

Dick
Homework Helper
Um. The radius and the mass are correct. But I get 9.4km/sec. Check the decimal point.

Dick
Homework Helper
$$g_e = \frac{GM_{e}}{R_{e}}$$

$$\frac{1}{4}g_e = g_{distant planet}$$

$$\frac{1}{4}*\frac{GM_{e}}{R_{e}} = \frac{G*2M_{e}}{R_{distant planet}}$$

$$\frac{1}{4*R_{e}} = \frac{2}{R_{distant planet}}$$

$$R_{distant planet} = 8R_{e}$$
g=G*M/r^2. It's force/mass not potential/mass.

Yeah, looks like I was just being retarded

Dick