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Homework Help: Force between two charged particles

  1. Nov 19, 2013 #1
    1. The problem statement, all variables and given/known data

    Two particles with a similar charge are held at a distance of 3.2*10^-3 m and then released. The acceleration of the first particle is measured at 7.0 m/s^2, and for the second at 9.0 m?s^2. The mass of the first particle is 6.3*10^-7. What is the mass of the second particle?

    2. Relevant equations

    F=ma F=G*m1*m2/r^2

    3. The attempt at a solution

    F1=m1a1 F2=m2a2 (where F1 and F2 are the forces exerted by each particle respectively)

    F1 - F2 = G*m1*m2/r^2

    (since the particles are similarly charged, they would have forces pointing in opposite directions)

    after substitution and making m2 the subject of the formula:

    m2 = (m1*a1*r^2)/(Gm1 + a2r^2)

    which gives: m2 = 4.31*10^-7

    I am not so sure if the method I used is completely correct, can someone please tell me?
  2. jcsd
  3. Nov 19, 2013 #2


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    Staff: Mentor

    The particles are charged. Do you really think that they will primarily feel a gravitational attraction? And to you really need to calculate the value of the force, or isn't there one of Newton's law you can use?
  4. Nov 20, 2013 #3
    Hi, thanks for replying. Are the forces produced by the two particles equal? Because then I can use Newton's second law as such:


    Would this be correct?
  5. Nov 20, 2013 #4


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    Staff: Mentor

    Yes, this is waht I was hinting at. If particle 1 feels a force from particle 2, then particle two must feel an equal and opposite force from particle 1.
  6. Nov 20, 2013 #5
    In the second part they're asking me to find the charge of each particle. I know I should use this formula:

    F = k.q^2/r^2

    and solve for q. As my force should I substitute 2*ma, since there are two particles, or is one enough?
  7. Nov 20, 2013 #6


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    Staff: Mentor

    In essence, there are not two forces, but one that affects two particles, albeit in different directions. So no factor of two.
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