1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Conical pendulum with two charges

  1. Apr 16, 2015 #1

    The sistem above is the one I'm interested in. There is two equally charged spheres spinning on a plane. The line has L=[itex]\sqrt{2}[/itex] m and the spheres weight 0.6Kg. The angular speed is [itex]\omega[/itex] = 2rad/[itex]s[/itex].

    The radius for the circular trajectory is R=1m and so the centripetal force is [itex]F_{c}=m\omega^{2}R=2.4N[/itex].

    I must find the charges q so

    [itex]2.4=k\frac{q^{2}}{(2R)^{2}}[/itex], right? Where [itex]k=9*10^{9}N m^{2} C^{-2}[/itex].

    I found q=32.7[itex]\mu C[/itex] but the problem is said to have an answer 40[itex]\mu C[/itex].

    Where have I commited a mistake if any?

    Thank you.
    Last edited: Apr 16, 2015
  2. jcsd
  3. Apr 16, 2015 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Maybe you mistyped some numbers, but the centripetal force you calculated seems a couple of orders of magnitude off.
    Secondly, you show the spheres as having the same charge, not opposite charges, so they are not going to provide a force in the right direction.
    Thirdly, the diagram makes it look as though they are suspended from wires. Is that the case?
  4. Apr 16, 2015 #3
    I mistyped the angular velocity. I wrote 0.2... now it's corrected. It should be 2.

    They are repelling each other and yes they are suspended from wires. The component of the wire's tension pointing to the center of the circular trajectory balances the Coulomb force. This system is spinning in this configuration.
  5. Apr 16, 2015 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Ok, so draw yourself a free body diagram of one charge. What are the forces on it and which way do they act? What do the horizontal and vertical Fnet=ma equations tell you?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted