Question on Gravity Keplers Laws?

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Homework Help Overview

The discussion revolves around the application of gravitational laws, specifically in the context of an object dropped from a significant height above the Earth's surface. The original poster attempts to calculate the initial acceleration using gravitational equations, referencing Kepler's laws and the gravitational constant.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the appropriate use of the gravitational formula, questioning the correct value for the radius in relation to the center of the Earth. There is also a focus on ensuring the correct units are used for the gravitational constant.

Discussion Status

Some participants have provided hints and clarifications regarding the use of the gravitational formula and the importance of considering the Earth's radius. The original poster indicates progress in their understanding after applying the correct radius in their calculations.

Contextual Notes

There is a mention of potential confusion regarding the units for the gravitational constant and the need for clarity on the mass representation used in calculations. The original poster's initial approach did not yield the expected results, prompting further exploration of the assumptions made.

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Question on Gravity Keplers Laws?

An object is dropped from a height of 1.27E+7m above the surface of the earth. What is its initial acceleration?

I thought I could use the equation for objects near the surface of the Earth in free fall which is :
g (or a) = GM / R^2
But this doesn't give me the right answer. Can anyone point me in the right direction?
 
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Welcome to PF!
You didn't use R=1.27E+7m, did you?
In that case, you'll get the wrong answer..
 
To give you a hint:
The "R" in the gravitation law is the DISTANCE FROM THE CENTER OF THE EARTH.
 
Becareful about what value you use for G.
The gravitational constant G comes in many flavors depending on your units.
You've used meters to represent the radius, and I'm not sure if you're expressing mass in g, kg, or Earth masses. So make sure you pick the right G, or convert your units for use with the G that you're using.
 
I got it! I added the Earth's radius (6.37E+6) to the radius above the Earth's surface and used it in my equation to get the right answer (1.09). Thank you very much for the help!
 

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