Two positively charged particles, one twice as massive as the other, are moving in the same circular
orbit in a magnetic field. Which law explains to us why the less massive particle moves at twice the
speed of the more massive particle?
(A)Coulomb’s Law
(B)Conservation of angular momentum
(C)Hooke’s Law
(D)The ideal gas law
(E)Heisenberg’s uncertainty principleThe hasty student will notice that the question is about charged particles, and see “Coulomb’s
Law” as the first answer choice. Without further ado, the student answers A and loses a quarter of
a point.
A more careful student will not just read the question, but will take a moment to understand the
question before glancing at the answer choices. This student will realize that the question
ultimately deals with particles moving in circular orbits, and the relative speeds of these particles.
Whether or not these particles are charged is irrelevant: you’re facing a problem of rotational
motion, not of electric forces. Once you’ve recognized what you’re dealing with, you will have
little trouble in correctly answering B.
General Hint 6: Know How to Guess
ETS doesn’t take off^1 / 4 of a point for each wrong answer in order to punish you for guessing.
They do it so as not to reward you for blind guessing. Suppose that, without looking at the
questions at all, you just randomly entered responses in the first 20 spaces on your answer sheet.
Because there’s a 20% chance of guessing correctly on any given question, odds are you would
guess right for four questions and wrong for 16 questions. Your raw score for those 20 questions
would then be:.
You would be no better off and no worse off than if you’d left those twenty spaces blank.
Now suppose in each of the first 20 questions you are able to eliminate just one possible answer
choice, so that you guess with a 25% chance of being right. Odds are, you’d get five questions
right and 15 questions wrong, giving you a raw score of:.
The lesson to be learned here is that blind guessing doesn’t help, but educated guessing does. If
you can eliminate even one of the five possible answer choices, you should guess. We’ll discuss
how to eliminate answer choices on certain special kinds of questions in Physics Hint 5:
Eliminate Wrong Answers.
Guessing as Partial Credit
Some students feel that guessing is like cheating—that guessing correctly means getting credit
where none is due. But instead of looking at guessing as an attempt to gain undeserved points, you
should look at it as a form of partial credit. Suppose you’re stumped on the question we looked at
earlier regarding the charged particle moving in circular motion in a magnetic field. Though you
don’t know the correct answer, you may know the answer isn’t the ideal gas law, because the
question doesn’t deal with gases in any way. Suppose you also know that the answer isn’t Hooke’s
Law, because Hooke’s Law deals with force exerted by a spring, and there are no springs in this
question. Don’t you deserve something for that extra knowledge? Well, you do get something:
when you look at this question, you can throw out C and D as answer choices, leaving you with a
one in three chance of getting the question right if you guess. Your extra knowledge gives you
better odds of getting this question right, exactly as extra knowledge should.