# Understand Newton's law of universal gravitation

1. Oct 25, 2008

### aligass2004

1. The problem statement, all variables and given/known data
http://i241.photobucket.com/albums/ff4/alg5045/1012540B.jpg

Consider the earth following its nearly circular orbit (dashed curve) about the sun. The earth has mass 5.98 *10^24 kg and the sun has mass 1.99 *10^30 kg. They are separated, center to center, by r=93 million miles=150 million km. What is the size of the gravitational force acting on the earth due to the sun?

2. Relevant equations

F = G(mE)(mS)/r^2

3. The attempt at a solution

Pretty much I just plugged and chugged to try to get the answer, which of course of was incorrect.

F = (6.67*10^-11)(5.98*10^24)(1.99*10^30)/ (1.5*10^11)^2 = 3.53*10^66

2. Oct 25, 2008

### alphysicist

Hi aligass2004,

I think you have a calculator error here. If you multiply these numbers together, you don't get an exponent of 66.

3. Oct 25, 2008

### aligass2004

4. Oct 25, 2008

### alphysicist

What is the statement of the problem?

I'm assuming re is not the radius of the earth, or you could just calculate that quantity from what you already have. Are the forces cancelling on that object, or is something else happening?

Last edited: Oct 25, 2008
5. Oct 25, 2008

### aligass2004

r is the radius of the earth. I still just don't understand what it's asking for.

6. Oct 25, 2008

### alphysicist

If you can, please post the entire problem (all parts, even those that you may not have gotten too yet) exactly as its written, and maybe I or someone else can help you interpret it.

If all you need is the gravitational constant G times the mass of the earth divided by the radius of the earth squared, you can find those quantitites easily (they are usually inside the covers of the first-year physics textbooks).

7. Feb 17, 2009

### stoked2skate

"Now it asks for the value of the composite constant (GmE/r^2), to be multiplied by the mass of the object mO in the following equation?"

The solution to the composite constant is simply 9.8 m/s^2.

Convert the radius of 6.38*10^3 km to meters... which is 6,380,000 m
(GmE/r^2)= (6.67*10^-11)(5.98*10^24)/(6,380,000)^2

Calculate this and you will get 9.81 m/s^2