# Acceleration due to gravity for a hypothetical planet

#### balletgirl

1. Homework Statement
A hypothetical planet has a radius 2.5 times that of Earth, but has the same mass. What is the acceleration due to gravity near its surface?

R(earth)= 6.38 x 10^4 m
M(earth)= 5.98 x 10^24 kg
G= 6.67 x 10^-11

2. Homework Equations
g=GM/r^2

3. The Attempt at a Solution
I did: (6.67 x 10^-11)(5.98 x 10^24kg) / [2.5(6.38 x 10^4m)]^2
and got 15,708, but know this is wrong since the answer is near only 15.

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#### shallgren

The radius of earth that you have looks like it's supposed to be kilometers.

Edit: Actually, no because earth's radius is actually 6378 km so the exponent is just messed up.

#### balletgirl

Okay I just looked it up, the radius in km is 6.38 x 10^4, and the radius in meters is 6.38 x 10^6.

I used this instead and it worked! Thank you for pointing this out for me, and the answer was actually 1.5 m/s^2, not 15.

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