# Gravity between 2 objects touching

1. Jun 5, 2009

### talaroue

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

2. Relevant equations
F=Gm1m2/r^2
138.07=M1+M2

3. 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?

2. Jun 5, 2009

### D H

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

3. Jun 5, 2009

no problem

4. Jun 5, 2009

### talaroue

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

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

(138.07+/-51.5012)/2

then i get 94.78, and 87.058.

5. Jun 5, 2009

### D H

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

6. Jun 5, 2009

### 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.

7. Jun 5, 2009

### 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
...would really be

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

Last edited: Jun 5, 2009
8. Jun 5, 2009

### talaroue

Last edited by a moderator: Apr 24, 2017
9. Jun 5, 2009

### D H

Staff Emeritus
What does this have to do with C++?

10. Jun 5, 2009

### 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

11. Jun 5, 2009

### D H

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

12. Jun 5, 2009

### talaroue

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

13. Jun 5, 2009

### D H

Staff Emeritus
Did you try 94.8?

14. Jun 5, 2009

### talaroue

yes, and it was wrong.

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