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Homework Help: Electric Force and Object Acceleration

  1. Sep 26, 2010 #1
    1. An object with a charge of -2.9 microC and a mass of .012 kg experiences an upward electric force, due to a uniform electric field, equal in magnitude to its weight.

    Part A: Find the magnitude of the electric field. Which I correctly calculated to be 4.1 x 10^4 N/C.

    Part B: If the electric charge on the object is doubled while its mass remains the same, find the direction and magnitude of its acceleration.



    2. F=ma



    3. I'm not really sure how to calculate the acceleration of the problem. Any ideas?
     
  2. jcsd
  3. Sep 26, 2010 #2

    Doc Al

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    Staff: Mentor

    What happens to the electric force if the charge is doubled? What happens to the net force on the object?

    ΣF = ma
     
  4. Sep 26, 2010 #3
    is the net force doubled as well?
     
  5. Sep 26, 2010 #4

    Doc Al

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    What was the net force originally?
     
  6. Sep 26, 2010 #5
    Net force was F=mg so F=(.012)(9.81)= .11772 N
     
  7. Sep 26, 2010 #6
    you have two forces, gravity and the electric force due to the electric field.

    You need to sum the all of the forces in your system (in the Y direction)

    Electric Force = q*E

    sum of the forces = 2qE - mg = ma; //solve for a

    If the acceleration is greater in than gravity, than the direction is obviously ......
     
  8. Sep 26, 2010 #7
    Ok, well a = [2(-2.9x10^-6)(4.1x10^4) - (.012)(9.81)]/.012 = -29.6 m/s^2

    But this is not the correct answer according to Mastering Physics.
     
  9. Sep 26, 2010 #8

    Doc Al

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    No, mg is just one of the forces on the object. What's the other? What's the net force?

    Hint: To solve part B you won't need the result from part A.
     
  10. Sep 26, 2010 #9
    Oh..I get it. So it would be 2q -mg= ma, so the answer would be 9.81 m/s^2 & it's positive because the force is upward. Thank you so much!
     
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