• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Using a pendulum to find Planetary mass and radius

Long Time reader, first time caller

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

An explorer wants to find planetary mass M and radius R using a pendulum of length L. The average elevation of the planet is R. At this elevation the pendulum has period T1. He then climbs up 2km where the pendulum has period t2. Express M and R interms of other variables need to calculate. Assume the planet is spherical

2. Relevant equations

Fg= GMm/R^2


3. The attempt at a solution

g = 4pi^2 L/T^2= GMm/R^2

I know I need another equation, I was hoping someone could point one out or tell me what Im missing. Obviously this could be solved one of ther variable but at the moment I have 3 var. and 2 equations. I thought about CoM but or CoE but then that brings in a lot more variables.

any thoughts


Homework Helper
4pi^2 L/T^2= GM/R^2 (note: NOT GMm/R^2) is for ground level. Write another equation that says the pendulum has period t2 2km up.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving