Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Projectile Motion involving eletric fields

  1. Jan 23, 2007 #1
    1. The problem statement, all variables and given/known data

    Ok, so, I've about had it rattling my brain on this. I'm stumped.


    Protons are projected with an initial speed vi = 9.89x10^3 m/s into a region where a uniform electric field E = (-720 j) N/C is present. The protons are to hit a target that lies at a horizontal distance of 1.27 mm from the point where the protons cross the plane and enter the electric field.

    Find the two projection angles that result in a hit and the corresponding times of flight.

    2. Relevant equations

    The only thing I was able to compute (hopefully I got it right) was that the acceleration in the y-direction (as there is none in the x) is -6.89687x10^10 m/s^s. Since E is -720j and knowing the charge and mass of a proton I calculated that... but that's about all I've got.

    I know all of my kinematic equations but I'm still at a loss on this problem...

    3. The attempt at a solution

  2. jcsd
  3. Jan 23, 2007 #2


    User Avatar
    Homework Helper

    What is the the net vertical distance travelled by the proton before hitting the target?

    Will this help to find time elepsed?

    What is the horizontal distance travelled during this time?
  4. Jan 23, 2007 #3
    The horizontal distance is 1.27mm... but there are two vertical distances depending on the angle theta...
  5. Jan 24, 2007 #4
    decompose the problem into x and y direction... write down the equation of motion in terms
    of the angle theta and time t...

    For the x direction, the distance travel is 1.27mm... v_x depend on the angle theta and the
    projection velocity... it is easy, you should have no problem on this one....
    For the y direction, vertical distance travel is zero... right?

    Now you have two equation with two unknown...
  6. Jan 24, 2007 #5
    The equations I have are:

    dx = vxit
    vyf = vyi + at
    dy = vyit + (1/2)at^2

    However, t is unknown and vyi and vxi are all dependent on theta...
  7. Jan 24, 2007 #6
    so, you have
    [tex] d_x = v_x^i t [/tex]
    [tex] d_y = v_y^i t + \frac{1}{2} at^2 [/tex]

    and from the relationship

    [tex] v_x^i = v^i cos\theta [/tex]
    [tex] v_y^i = v^i sin\theta [/tex]

    The only unknown(s) are t and [tex] \theta [/tex]
    Could you solve for [tex] \theta [/tex] by eliminating t??
  8. Jan 24, 2007 #7
    But then wouldn't I be left with something like:

    [tex]d_x = v^i cos\theta t[/tex]

    Which would result in:

    [tex]t = \frac{d_x}{v^i cos\theta}[/tex]

    And after using [tex]sin^2\theta + cos^2\theta = 1[/tex] I should be able to use a quadratic formula to get two values of t... but I only end up getting one and it doesn't result in a correct answer.
  9. Jan 24, 2007 #8
    Why would you use [tex]sin^2\theta + cos^2\theta = 1[/tex] ???

    since you know [tex]t = \frac{d_x}{v^i cos\theta}[/tex], just plug it into
    [tex] d_y = v^i sin\theta t + \frac{1}{2} at^2 [/tex]

    with the t eliminated, now you are left with one equation with one unknown...
    Everything shall be crystal clear from here, right?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook