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 · m^2 ⁄ kg^2 , what is the best approximation
for the radius of the satellite’s orbit?
(A)
m
(B)
m
(C)
m
(D)
m
(E)
mWhat’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.
Slightly Underhanded Way #1: Elimination through Logic
By scanning the possible answer choices, you can see that the answer will begin either with a 4 or
a 2.5. There are three options beginning with 4 and only two beginning with 2.5. Odds are, the
correct answer begins with 4. The test makers want to give you answer choices that are close to
the correct answer so that, even if you’re on the right track, you might still get caught in a