# Good physics problem

1. Jun 19, 2013

### jingu

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

There are two roads of length x.There separation is also x.They have mass k and p.Than what is the force exerted by one on the other.

2. Relevant equations

I think f=gmM/R^2 IS THE EQUATION TO BE USED.
3. The attempt at a solution
When I solved (dx)^2 was coming during integration which is not solvable for me.

Last edited by a moderator: Jun 19, 2013
2. Jun 19, 2013

### voko

You have not shown us how you end up with your problem, so we cannot help.

3. Jun 19, 2013

### Tsunoyukami

Please show exactly what you did so we can see where you went wrong. :)

4. Jun 19, 2013

### jingu

all i did is this....

Let a small element of road 1 be dy.Then dm1= (m1/x)dy.Similarly dm2=(m2/x)dy.
dF=(G(dm1)(dm2))/y^2
dF=(Gm1*m2*(dy)^2)/y^2
∫dF=∫(Gm1*m2*(dy)^2)/y^2
here the problem begins we have to integrate from x to 2x.WRT to (dy)^2...now can we integrate this.... how to solve with (dy)^2?......I think I am going in a wrong path.guide me.

5. Jun 19, 2013

### voko

Do not use 'y' for both dm1 and dm2. Use different symbols, say 'u' and 'v'.

6. Jun 19, 2013

### jingu

I am getting confused there are so many variables...... Please give me the description of how to approach the problem.Everything else will be my work.I have to submit it tommorow.

Last edited: Jun 19, 2013
7. Jun 19, 2013

### voko

$$dm_1 = \frac {m_1} {x} du \\ dm_2 = \frac {m_2} {x} dv$$

8. Jun 19, 2013

### CAF123

You are considering the force $dF$ by the infintesimal mass element $dm_1$ on one rod (road?) on another infintesimal element $dm_2$ on the other. Then summing up all the contributions. Consider some mass element (with fractional length $du/x$) on one and another mass element (of fractional length $dv/x$) on the other.

9. Jun 19, 2013

### jingu

dF=(Gm1m2dvdu)/x^2*r^2 here r is the distance between them ,then r equals what....?This question would be easy if there is a point mass and a continuous body...but here both are continuous....?
Then this need to integrated but with respect to what..........?
Please show me the calculations .....I am studying this topic for first time.......help....I am trying this question since 2 days....

10. Jun 19, 2013

### HallsofIvy

If you have an integrand involving "du" and "dv", what do you think it should be integrated with respect to?

11. Jun 19, 2013

### voko

The integration variables are u and v, which should be obvious from 'du' and 'dv'.

What is the topic you are studying?

12. Jun 19, 2013

### jingu

Various types of forces....

13. Jun 19, 2013

### voko

But your difficulty has nothing to do with the topic. You are having problems with integration, which is basic calculus.

14. Jun 19, 2013

### jingu

I am not able to understand what u all are telling............?explain in detail...I am in 10 grade so Iam not familiar with calculus....Only very basic I know....

15. Jun 19, 2013

### voko

Grade 10? Hmm. I do not think you have studied double integrals then.

Do you know what a potential of force is?

16. Jun 19, 2013

### jingu

NO.That's why I posted it here, i hoped a solution to this problem...but u r not helping me....Give me a full solution so that i can understand it thoroughly... ....

17. Jun 19, 2013

### barryj

Can you not consider the road a point mass of k and p separated by x?

18. Jun 19, 2013

### jingu

I dont think so......I am expecting a solution but no one is ready to give......

19. Jun 19, 2013

### voko

You will not get a solution in this forum, it is against the rules.

20. Jun 19, 2013

Look again at the suggestion given by barryj in post 17. I suspect that this is the approach expected for someone in tenth grade.