1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    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!

Homework Help: Static system + a second long distance force

  1. Oct 7, 2008 #1
    1. The problem statement, all variables and given/known data

    When an electrically charged object (a proton, an electron, a balloon after you’ve rubbed it
    on your hair) is placed in an electric field, E , an electric force proportional to this field acts
    on the object. Specifically,

    F = qE

    where q is the charge on the object and is in units called Coulombs (C).


    The figure shows a pendulum, the bob of which is charged. The ambient electric field is
    uniform, directed toward the right, and has a magnitude of 2 × 10^5 N/C. The pendulum
    hangs at an angle of 20 degrees as a result of the forces acting on it. What is the charge on the bob of the pendulum? The string can be considered of negligible mass and uncharged.


    (theta) = 20 degrees
    E = 2 x 10^5 N/C
    m = 5.0g (or 0.005kg)
    a(g) (acceleration due to gravity) = -9.8m/s
    F(e) (force of electric... thing) = ?
    T(xy) (x and y component of tension in rope) = ?
    F(net) (net force) = 0

    2. Relevant equations

    F = ma
    F = qE
    probably like four I can't remember

    3. The attempt at a solution

    I know that F(net) is zero, 'cause it's a static system. So:

    F(net) = T(xy) - F(e) - F(g) = 0

    The force of gravity is 0.049N (0.005kg*9.8m/s^2).

    But I have no idea how to find the tension in the rope. If I could get it, the only thing left would be to solve for F(e) using the net force equation, and then rearrange the F(e) = qE equation to solve for q, but that's the easy part.

    The second long-distance force is throwing me off. Also, I forget all of high school trig (it's been like, four years) and even then I'm not sure how I would apply it. cos(theta)*T(xy) = T(y), sin(theta)*T(xy) = T(x) (which, I'm kind of grabbing at straws here, so I give myself a 50/50 chance on being right). So what - I can't see a way for me to get or apply either of those. I'm stuck. :<

    Thanks for reading.

    EDIT: Is the magnitude of T(x) just the same as F(g)? If it is I'm gonna be mad, 'cause this took way too long. :'(
    Last edited: Oct 7, 2008
  2. jcsd
  3. Oct 7, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi imzkris! Welcome to PF! :smile:

    You could find T by taking vertical components …

    but you don't need to find T …

    when there's an unknown force in a known direction, like T, just take components perpendicular to that unknown force. :wink:
  4. Oct 7, 2008 #3
    I'm... not following. :shy:
  5. Oct 7, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    ok … the tension force, T, is along the string, at 20º to the vertical.

    So take components perpendicular to that (at 20º to the horizontal).

    Then the component of T in that direction will be Tcos90º = 0,

    and the components of the other forces are … ? :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook