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Homework Help: Electric Forces in 1D Motion

  1. Jan 18, 2016 #1
    1. The problem statement, all variables and given/known data
    In outer space, a ball with mass 0.25kg and charge +4/K C is shot from 5m towards a +0.5C charged ball with an initial velocity of 20 m/s
    1. What main Physics principle should be used to solve this problem?
    2. What is the closest the small ball will get to the large one?
    3. What is the velocity and acceleration at this position?
    2. Relevant equations
    F(electric) = KQq/r^2 where K = 9*10^9 Nm^2/C^2

    3. The attempt at a solution
    1. I should use Newton's Second Law because by using F = ma, I could find a, then substitute into the 1D equation for displacement to solve for how far the 0.25kg ball travels.
    2. I got a=F/m = -0.32 m/s^2 and substituted into the equation vf=v0+2ax (I made vf=0) to find x. My answer was 625 m which is not reasonable.
    3. I couldn't find the answer without a correct answer from the second question.
  2. jcsd
  3. Jan 18, 2016 #2


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    2017 Award

    It is not clear whether the second ball is free to "recoil" or is somehow fixed in space.

    Not sure what the units /K C means.

    0.5 C is a HUGE amount of charge for a ball to have. (That's enough charge to blow the ball apart.)

    Do you expect the acceleration of the 0.25 kg ball to remain constant as it approaches the other ball?

    The equation vf=v0+2ax is not correct. It looks similar to an equation that is only valid for constant acceleration.
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