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Homework Help: Coulomb's Law involving triangle

  1. Mar 23, 2013 #1
    1. The problem statement, all variables and given/known data

    There are three point masses. 1 is fixed in space with the 2nd point mass directly below it on the ground. The 3rd point mass is an unknown distance to the right of mass number 2. These 3 point masses for a rt triangle with point 2 at the 90 degree angle. The vertical distance between point mass 1 and 2 is 1.5 m. The mass of each point mass is 5.5kg. Also there is kinetic friction between point mass 2 and the ground. All three point masses have a charge of +15 Coulombs. If mass 3 starts moving toward mass 2, how far away from 2 will 3 be when 2 starts to move?

    2. Relevant equations


    This problem seems to combine coulomb's law with with 2-d motion. However, I am stumped as to how to combine the 2 concepts. also, not knowing the original starting distance between 3 and 2 is complicating matters.

    3. The attempt at a solution

    I am able to calculate the force that 1 exerts on 2 no problem but have no idea how to figure out the force that 3 exerts on 2. i also have no idea how to incorporate friction
  2. jcsd
  3. Mar 23, 2013 #2

    Simon Bridge

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    It is the same as normal only now you know the origins of the forces.
    Draw a free body diagram for mass 2.
  4. Mar 24, 2013 #3
    I see what you mean. So the force acting on 2 is equal to the frictional coefficient*(mg + force of 1 acting on 2), right.

    I'm thinking I use the conservation of energy equation to determine how far 3 is from 2 when 2 starts to move, is that right?
  5. Mar 24, 2013 #4

    Simon Bridge

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    How does conservation of energy help here?
    Why not just add up all the forces - you know what they have to sum to?
    Keep the variables (it helps that all the masses and charges are the same) and solve for the distance to charge 3.
  6. Mar 24, 2013 #5
    I am a little confused still. I have calculated the original distance between 2 and 3 before 3 starts to move from the right. Does 2 start to move at same time 3 starts to move, if so then the original starting distance is the distance from 2 that I am solving for.
    Last edited: Mar 24, 2013
  7. Mar 24, 2013 #6

    Simon Bridge

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    Of course not :)
    3 starts out so far away that the friction is enough to hold 2 where it is.
    As 3 approaches 2, the coulomb force increases - at some point the force will be so large that the friction no longer holds 2 in place.

    All the problem wants to know is how close 3 can get to 2 without 2 moving.
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