美国高考SATII物理专题分析--SAT II Physics Test-Taking Strategies(3)
If you’re wondering, the answers to the above questions are 1 A, 2 E, and 3 D.
“EXCEPT” Questions
All of the following are true about an -particle EXCEPT
(A) It has an atomic mass of 4
(B) It carries a positive charge
(C) It is identical to the nucleus of a helium atom
(D) It will always pass right through a thin sheet of gold foil
(E) It contains two neutrons
Questions of the “EXCEPT” variety contain a bunch of right answers and one wrong answer, and it’s generally possible to spot one or two right answers. Even if you can’t answer the question confidently, you might remember that alpha particles have a positive charge and that they are identical to the nucleus of a helium atom. Already, you’ve eliminated two possible answers, and can make a pretty good guess from there.
If you’re interested, the answer is D: Rutherford’s gold foil experiment showed that alpha particles would occasionally deflect off the gold foil at extreme angles, thus proving that atoms have nuclei.
“I, II, and III” Questions
For which of the following is f > 0:
I. Concave mirror
II. Convex mirror
III. Converging lens
(A) I only
(B) II only
(C) I and III only
(D) II and III only
(E) I, II, and III
In this style of multiple-choice question, the “I, II, and III” questions provide you with three possible answers, and the five answer choices list different combinations of those three. There’s an upside and a downside to questions like these. Suppose you know that a concave mirror has f > 0 and a convex mirror doesn’t, but you’re not sure about a converging lens. The downside is that you can’t get the right answer for sure. The upside is that you can eliminate B, D, and E, and have a 50% chance of guessing the right answer. As long as you’re not afraid to guess—and you should never be afraid to guess if you’ve eliminated an answer—these questions shouldn’t be daunting.
The value of f for a converging lens is positive, so the answer is C.
Physics Hint 6: Be Flexible
Knowing your physics formulas is a must, but they’re useless if you don’t know how to apply them. You will probably never be asked to calculate the force acting on an object given its mass and acceleration. Far more likely, you will be asked for the acceleration given its mass and the force acting on it. Knowing that F = ma is useless unless you can also sort out that a = F⁄m.
The ETS people don’t want to test your ability to memorize formulas; they want to test your understanding of formulas and your ability to use formulas. To this end, they will word questions in unfamiliar ways and expect you to manipulate familiar equations in order to get the right answer. Let’s look at an example.
A satellite orbits the Earth at a speed of 1000 m⁄s. Given that the mass of the Earth is
kg and the universal gravitational constant is 
N · m2 ⁄ kg2, what is the best approximation for the radius of the satellite’s orbit?
(A) 
m
(B) 
m
(C) 
m
(D)
m
(E) 
m
What’s the universal gravitational constant? Some people will know that this is the G in the equation for Newton’s Law of Gravitation: 
. Other people won’t know that G is called the “universal gravitational constant,” and ETS will have successfully separated the wheat from the chaff. It’s not good enough to know some formulas: you have to know what they mean as well.
Given that we know what the universal gravitational constant is, how do we solve this problem? Well, we know the satellite is moving in a circular orbit, and we know that the force holding it in this circular orbit is the force of gravity. If we not only know our formulas, but also understand them, we will know that the gravitational force must be equal to the formula for centripetal force,
. If we know to equate these two formulas, it’s a simple matter of plugging in numbers and solving for r.
Knowing formulas, however, is a small part of getting the right answer. More important, you need to know how to put these two equations together and solve for r. On their own, without understanding how to use them, the equations are useless.
But there are two slightly underhanded ways of getting close to an answer without knowing any physics equations. These aren’t foolproof methods, but they might help in a pinch.