Gravity between 2 objects touching

[talaroue]

Homework Statement

Two spherical objects have a combined mass of 138.07 kg. The gravitational attraction between them is 1.48×10-6 N when their centers are 43 cm apart. What is the larger mass?

Homework Equations

F=Gm1m2/r^2
138.07=M1+M2

The Attempt at a Solution

F=1.48X10^-6
G=6.67X10^-11
r=.63 m
m1 and m2 is what we are solving for.

so i used the second equation and solved for M1. and plugged it into the first equation for M1.
F*r^2/G=(Mtot-M2)

then i solved for M2 and got a quadratic equation. m^2 -138.07m+4102.728. then used the quadratic equation. and came up with 86.568. What is wrong with my logic or math?

 

[D H]

Please show your work on how you used the quadratic equation.

 

[talaroue]

no problem

 

[talaroue]

-b+/- sqrt(b^2-4ac)/2a

138.07+/-sqrt(138.07^2-(4*1*4102.728))/2

(138.07+/-51.5012)/2then i get 94.78, and 87.058.

 

[D H]

(138.07+51.5012)/2 is 94.78.
(138.07-51.5012)/2 is not 87.058.

 

[talaroue]

94.78 didnt work maybe his C++ language is wrong...I think its wrong for another one I am about to post. For some reason gravity is not making any since to me, everytime i head in a direction I get an anwser that ends up wrong. ill post a link after I make it.

 

[LowlyPion]

Here is a quadratic calculator that you might find useful:
http://www.math.com/students/calculators/source/quadratic.htm

I would concur with the 94.79 number.

As to your method it seems fine to me.

Though I think your expression left off your factor of M2

F*r^2/G=(Mtot-M2)

...would really be

F*r/G = (138.07 - m)*m

 

[talaroue]

ok so i am starting to understand. Here is another problem that I think the C++ language is wrong.

https://www.physicsforums.com/showthread.php?t=318235"

 

[D H]

What does this have to do with C++?

 

[talaroue]

Oh the homework I have to submit online and it tells me whether or not my answer is right or not. It is saying that 94 is wrong. So I am thinking that the C++ language he used to formulate the anwser is wrong because he created it himself

 

[D H]

Why did you enter 94? You know that isn't the answer.

 

[talaroue]

You get multiple tries at each question, that is how I found out that 94.78 is wrong.

 

[D H]

Did you try 94.8?

 

[talaroue]

yes, and it was wrong.