i have a few problems that i was struggling with.(adsbygoogle = window.adsbygoogle || []).push({});

Projectile motion:

An electron is fired in an electric feild E=-720j. Its launched with a velocity of 9550 m/s. if you want the x distance it travels to be 1.27mm, what are the two values of theta that will work? how long does it take?

so, like i do all projectile problems, i found t using the x components, (which i left in terms of theta,) because there's no force in x. then i plugged t into y_f= 1/2at^2 +v_ot. my problem was that i was left with cos and sin's and i didn't know what to do with them. i couldn't easily solve for theta. and at any rate, i wasn't really sure how to use the electric feild. when i thought about gravity fields, i thought that i should be using energy or something, and i just got confused.

SHM of charge ring:

i forget the wording of this one, but its just like, show that the frequency of a particle moving on the axis of a ring is [tex]f=\frac{1}{2\pi}*(\frac{kQq}{ma^3})^1/2[/tex]

i tried starting with the electric feild, and getting the force from that. when my prof did an example with the spring force in class, he did this really weird thing i can't follow now. he was like f=1/s and so if F=kx=ma then k/m*x=d^2x/dt^2, and what function looks like its second derivative? sin. and then we have the constant k/m, which must be frequency squares since sin(ax)'s second deriv is a^2sin(ax). and thats how he found the frequency for the spring. i have NO idea how to do that with the electric force. i tried working backwards too, but i couldn't really make sense of it because there's x's in the denominator of the force and not the frequency, and yeah, i'm stuck.

The last problem i'm stuck on is this-

Electric quadrapole:

whats the electric feild along the y axis if there's a quadrapole on the x axis, with charge q at (-a,0), -2q at (0,0) and q at (a,0)?

so i started with writing the feild generated by each. knowing that the x components of the two q charges would cancel, and their y's would be equal. so i got two terms, and i simplified and used the binomial theorem, and i couldn't get it right. but i think i'm going to try this one again.

thanks!

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# Homework Help: E&m problems

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