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Homework Help: General physics contest problems please help

  1. Mar 20, 2007 #1
    1. The problem statement, all variables and given/known data
    A proton sits at coordinates (x; y) = (0; 0), and an electron
    at (d; h), where d >> h. At time t = 0, a uniform electric
    field E of unknown magnitude but pointing in the positive y
    direction is turned on. Assuming that d is large enough that
    the proton-electron interaction is negligible, the y coordinates
    of the two particles will be equal (at equal time)
    (a) at about y = d=2000;
    (b) at an undetermined value since E is unknown;
    (c) at about y = d=43;
    (d) nowhere: they move in opposite directions.


    2. Relevant equations
    qE=F=ma


    3. The attempt at a solution
    I chose b, but the answer is a


    Q2 Question 20
    TheWebb space telescope, scheduled to be launched in 2010,
    will have a mirror 6m in diameter. Compared with the Hubble
    space telescope, whose mirror has a 2:4m diameter, it will
    be able to resolve objects whose angular separation is about
    (a) 2.5 times smaller;
    (b) 5 times smaller;
    (c) an order of magnitude smaller;
    (d) the same: the larger mirror only increases the amount
    of light gathered.

    (no idea of telescope)

    Q3
    A person pulls a box along the ground at constant speed. Considering
    the Earth and the box together as a system, which of
    the following is true about the net force F exerted by the person
    on this system and the work W she does on it?
    (a) F = 0 and W = 0 ; (b) F != 0 and W = 0 ;
    (c) F = 0 and W != 0 ; (d) F != 0 and W != 0 .

    These are not hw problems, just contest prep. It would be great if someone can give me a sol'n as i have spend some time thinking about the problems already.

    Thanks!
     
  2. jcsd
  3. Mar 29, 2007 #2
    q1, oops:blushing: , i don't understand the choices, what's "y=d=2000"?:confused:

    q2. theta~=sin(theta)=lambda/a, a is the diameter of the lens. theta is the seperation angle. so 2.5 times

    q3. F=0, W=0 since she is exerting forces on earth and the box at the same time and the two forces are of same magnitude but opposite direction. draw a fbd and you'll get it.:smile:
     
  4. Mar 29, 2007 #3
  5. Mar 29, 2007 #4
    I have no idea whatsoever about telescopes. Could you go a bit step by step for #2? Thanks.

    For #3, the answer is actually C, which confuses me too
     
  6. Mar 29, 2007 #5
    heck I'm still puzzled why the answer to 1) isn't d. Can someone help me?
     
  7. Mar 29, 2007 #6
    If an electric field E points in the positive x-axis, then proton will be moving up and electron down. They will meet
     
  8. Mar 29, 2007 #7
    sorry postive y-axis
     
  9. Mar 29, 2007 #8
    q3 is tricky, sorry for the mistake, W!=0 since the person does little work on the ground (or idealistically 0 work has been done on the ground). Why? think about it, the person is like walking so the friction exerted by his/her feet is discontinuous.

    for q2, always try wikipedia first, there're plenty of info which will help u understand it much better. good luck on CAP ^_^
     
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