AP Physics C 2017

25 . A circular loop of wire in the plane of the page is placed in a magnetic field pointing into the page, as
shown above. Which of the following will NOT induce a current in the loop?
(A) moving the wire to the right in the plane of the page
(B) increasing the area of the loop
(C) increasing the strength of the magnetic field
(D) rotating the wire about a diameter
(E) turning the magnetic field off

END OF  DIAGNOSTIC  TEST

Answers and Explanations

1   .   D   —Choices    A   and E   don’t   make    sense   because the direction   of  an  object’s    acceleration    or  velocity

is  essentially arbitrary—when  solving a   problem,    you can usually pick    the “positive”  direction   based

on  convenience.    So  neither value   must be negative.   We  can rule    out choice  B   because we  know    that    a

fast    moving  object  can slow    down    very    gradually.  And there’s no  reason  why you need    multiple    forces

to  make    an  object  slow    down,   so  that    gets    rid of  choice  C.

2   .   E   —When   an  object  is  thrown  in  the absence of  air resistance  near    the surface of  the Earth,  its

acceleration    in  the vertical    direction   is  always  g   ,   the acceleration    due to  gravity,    which   has a

magnitude   of  9.8 m/s^2   .   The acceleration    due to  gravity is  directed    down,   toward  the Earth.  So  the

ball’s  acceleration    is  −9.8    m/s^2   .

3   .   D   —The    vertical    component   of  a   vector  is  the magnitude   of  the vector  times   the sine    of  the angle

measured    to  the horizontal; in  this    case,   F   1    sin    θ   .

4   .   D   —Something  that    moves   in  a   straight    line    at  constant    speed   is  in  equilibrium.    So, the sum of  left

forces  has to  equal   the sum of  right   forces.

5   .   C   —On an  incline,    the weight  vector  parallel    to  the plane   goes    with    the sine    of  the plane’s angle.  g sin

40° is  an  acceleration,   not a   weight.

6   .   E   —Because    the weight  is  the only    force   acting  parallel    to  the plane,  mg sin  40° =   ma  ,   so  a = g sin

40°.    This    acceleration    is  down    the plane,  in  the direction   of  the net force.  Yes,    the block   is  moving  up

the plane,  but the block   is  slowing down,   and so  the acceleration    must    be  in  the opposite    direction

from    the velocity.

7   .   D   —All    forms   of  energy  are scalar  quantities: potential   energy, kinetic energy, work,   internal    energy

of  a   gas.    Energy  doesn’t have    direction.