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Homework Help: Force exerted by external field

  1. Jun 25, 2010 #1
    1. The problem statement, all variables and given/known data

    A uniform electrical field can exert a force on a charged particle that effectively neutralizes the force of gravity on the particle causing it to be weightless. What is the strength of the electrical field for a proton?

    q = 2.45x10^-14
    mass of proton = 10.21x10^-12 kg

    2. Relevant equations


    3. The attempt at a solution

    My question is what does that statement make the force?

    What I think it means is that gravity is 9.8 m/s^2 so to cancel it out gravity is -9.8m/s^2.
    So to sovle it you would use F=mg
    so, F = (-9.8m/s^2)(10.21x10^-12 kg)
    So Force = -1.00058x^-10
    Is that right?

  2. jcsd
  3. Jun 25, 2010 #2
    yes what you have there is essentially correct, just have to take it 1 step further

    For the proton to be weightless, the force exerted on the proton by the electric field has to be exactly equal to the force of Gravity.

    Force due to Electric Field
    F(e) = q * E

    Force due to Gravity
    F(g) = m * g

    So for the proton to be weightless, we have:
    F(e) = F(g)
    qE = mg
    Solve for E
    E = mg/q

    calculate E and you have the strength of the Electric Field required for the proton to be weightless
  4. Jun 25, 2010 #3
    Still counfused whether gravity is negative or positive?

    Everything else makes sense thanks.
  5. Jun 25, 2010 #4
    All you want is two forces to cancel.
    Force due to gravity is pointing downwards.
    So you want Fe to point upwards.
  6. Jun 26, 2010 #5
    It doesnt really matter if gravity is +ve or -ve
    The important thing to realise, as graphene said, is that the two forces are working in opposite directions. Gravity is pulling down, the Electric Field is pushing up.
    So for something to be weightless, the force pushing up has to be exactly the same as the force pulling down.

    The positive and negative is totally up to you. If you want to call the up direction positive then gravity is negative and the electric force is positive. If you want to call the down direction positive, then gravity is positive and the electric force is negative.

    As long as they are opposite and equal the sign doesnt matter because when you add them up, you will get a Net force of Zero and thats what makes the proton weightless.
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