AP Physics C 2017

Don’t   use the equation    sheet   to  “hunt   and peck.”  The sheet   can remind  you of  subtle  things; for example,

does    the magnetic    field   due to  a   wire    have    an  r or    an  r   2    in the denominator?    But if  you don’t   have    the

general idea    that    the magnetic    field   depends on  current and gets    weaker  farther away    from    a   wire,   then

you won’t   recognize

even    if  you go  hunting for it.

—Wyatt, college freshman    in  engineering

Some Examples

We mentioned in Step 2 that some questions on the AP exam are designed solely to test your knowledge of
equations. If you know your equations, you will get the question right. Here’s an example.

A   pendulum    of  length  L swings    with    a   period  of  3   s.  If  the pendulum’s  length  is  increased   to  2L  ,   what

will    its new period  be?

(A) 3/ s

(B) 3   s

(C) 3 s

(D) 6   s

(E) 12  s

The answer  is  (C).    The equation    for a   pendulum’s  period  is

since L is in the numerator and under the square root, multiplying L by 2 multiplies the period by .

Of course, the multiple-choice section will not be the only part of the exam that tests your knowledge
of equations. Often, a part of a free-response question will also test your ability to use an equation. For
example, check out this problem.

Four    charges +Q  are arranged    in  a   square  of  side    length  l   .

(a)     What    is  the magnitude   of  the electric    field   due to  just    one of  these   charges at  the center  of  the

square?

Yes,    later   in  the problem you’ll  be  asked   to  add vectors to  find    E due   to  a   bunch   of  charges.    Ugh.    But