(adsbygoogle = window.adsbygoogle || []).push({}); [SOLVED] Gravitational Potential Energy Problem

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

Suppose that the moon were at rest at its present distance from the earth, rather than orbiting it. With what speed would it strike the earth? (Take the earth to be infinitely massive relative to the moon)

2. Relevant equations

Mass of earth = 5.98x10^24 kg

Distance from center of earth to center of moon = R = 3.82x10^8 m

Mean radius of Earth = Re = 6.37x10^6 m

Gravitational potential energy = -GMm/R

K1+U1=K2+U2

3. The attempt at a solution

K1=0

k2=(1/2)Mmv^2

U1=-GMeMm/R

U2=-GMeMm/Re

(1/2)Mmv^2-GMeMm/R=-GMeMm/Re

when I eliminate the mass of the moon and plug in all the numbers to solve I get 11.10 km/s, but I know the right answer is 9.8 km/s. Anyone see what I am doing wrong?

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Gravitational Potential Energy Problem

**Physics Forums | Science Articles, Homework Help, Discussion**