<p>An electrically charged, insulated metal rod is observed to attract a neutral pith ball and, after contact is made, to repel the ball. Which of the following can be concluded about the rod?</p>
<p>Answer:
The rod's charge before and after contact had the same sign.</p>
<p>Can someone explain this to me?</p>
<p>bump 10char</p>
<p>Well you have a neutral pithball and either a positive or negative charged metal rod. if the pithball makes contact with the rod then the charge of both the objects are distributed equally. lets say the metal rod had a charge of -6 and the pith ball had 0. -6 and 0 attract. once the make contact, the metal rod will get a charge of -3 and it will give the pith ball a charge of -3. thats where you get your answer. and also heres another thing to know, its quite obvious:D. 2 negatives are NOT attacted to eachother. they repel.</p>
<p>Thanks. Anyone want to explain this one?
A proton of mass 1.7<em>10^-27 kilogram and speed 3</em>10^7 meters per second is moving in a circle of radius 200m. Which of the following is the best estimate of the order of magnitude of the magnetic force needed to maintain this motion?</p>
<p>Answer: 10^-14 N</p>
<p>For that one. F=kq1q2/r^2 and Centripedial force = mv^2/r. they can be set equal to eachother. so the F=mv^2/r. You plug in the numbers and it gives you 7.65x10^-15. thats the closest i can get. the electric field is 3.6x10^-14 but i dont think that means anything, its just close. the only thing i can think of is that they want you to pick the nearest answer</p>
<p>^Electrostatic force isn’t involved in this question.</p>
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<p>I think that the magnetic force is just centripetal force.</p>