General physics contest problems please help

  • Thread starter epl_manutd
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  • #1
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Homework Statement


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.


Homework Equations


qE=F=ma


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!
 

Answers and Replies

  • #2
43
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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
11
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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
 
  • #5
960
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heck I'm still puzzled why the answer to 1) isn't d. Can someone help me?
 
  • #6
11
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If an electric field E points in the positive x-axis, then proton will be moving up and electron down. They will meet
 
  • #7
11
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sorry postive y-axis
 
  • #8
43
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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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