- #1

- 11

- 0

## 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!