1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Good physics problem

  1. Jun 19, 2013 #1
    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. jcsd
  3. Jun 19, 2013 #2
    You have not shown us how you end up with your problem, so we cannot help.
  4. Jun 19, 2013 #3
    Please show exactly what you did so we can see where you went wrong. :)
  5. Jun 19, 2013 #4
    all i did is this....

    Let a small element of road 1 be dy.Then dm1= (m1/x)dy.Similarly dm2=(m2/x)dy.
    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.
  6. Jun 19, 2013 #5
    Do not use 'y' for both dm1 and dm2. Use different symbols, say 'u' and 'v'.
  7. Jun 19, 2013 #6
    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
  8. Jun 19, 2013 #7

    dm_1 = \frac {m_1} {x} du


    dm_2 = \frac {m_2} {x} dv

  9. Jun 19, 2013 #8


    User Avatar
    Gold Member

    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.
  10. Jun 19, 2013 #9
    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....
  11. Jun 19, 2013 #10


    User Avatar
    Science Advisor

    If you have an integrand involving "du" and "dv", what do you think it should be integrated with respect to?
  12. Jun 19, 2013 #11
    The integration variables are u and v, which should be obvious from 'du' and 'dv'.

    What is the topic you are studying?
  13. Jun 19, 2013 #12
    Various types of forces....
  14. Jun 19, 2013 #13
    But your difficulty has nothing to do with the topic. You are having problems with integration, which is basic calculus.
  15. Jun 19, 2013 #14
    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....
  16. Jun 19, 2013 #15
    Grade 10? Hmm. I do not think you have studied double integrals then.

    Do you know what a potential of force is?
  17. Jun 19, 2013 #16
    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... ....
  18. Jun 19, 2013 #17
    Can you not consider the road a point mass of k and p separated by x?
  19. Jun 19, 2013 #18
    I dont think so......I am expecting a solution but no one is ready to give......
  20. Jun 19, 2013 #19
    You will not get a solution in this forum, it is against the rules.
  21. Jun 19, 2013 #20
    Look again at the suggestion given by barryj in post 17. I suspect that this is the approach expected for someone in tenth grade.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Threads - physics problem Date
Heat capacities problem Monday at 3:19 AM
Physics Word Problem: Arrow and moving target collision Saturday at 10:55 PM
Finding the Mass of a Hanging Rope (Wave Problem) Friday at 6:38 PM
RC Circuit Problem Feb 25, 2018
Physics Vector Problem Feb 25, 2018