AP Physics C 2017

What if the answer choices had been like this:

(A) 2.5 m/s^2

(B) 4.9 m/s^2

(C) 9.8 m/s^2

(D) 19.6    m/s^2

(E) 78.4    m/s^2

Is the problem any different? (Answer: no.)

Concept Questions: “WHY?”

Many multiple-choice questions involve no calculations and no formulas. These test your understanding
of vocabulary and explanations for physical phenomena.

Two identical   train   cars    move    toward  each    other,  each    with    the same    speed   as  the other.  When    the train

cars    collide,    they    stick   together    and remain  at  rest.   Which   of  the following   fundamental physics

principles  can best    be  used    to  explain why the attached    cars    cannot  move    after   the collision?

(A)     Conservation    of  mechanical  energy

(B)     Conservation    of  linear  momentum

(C)     Conservation    of  angular momentum

(D)     Conservation    of  mass

(E)     Conservation    of  rotational  energy

The direct answer to this question is B: conservation of linear momentum applies to all collisions. The
cars had equal momentum in opposite directions, so the net momentum before collision was zero; thus, the
cars may not have any momentum after collision. Kinetic energy is a scalar, having no direction, and so
kinetic energy of two moving objects cannot cancel to zero. Mechanical energy was not conserved,
because kinetic energy was lost in the collision.
But even if you have a hesitation about the difference between momentum and kinetic energy
conservation, you can still get close to the right answer by eliminating obvious “stupidicisms.” Look at E:
perhaps you recognize that there’s no such thing as “conservation of rotational energy.” Or likely you see
right away that conservation of mass, while a legitimate concept, is usually relevant in a chemical process
or fluid dynamics and can have little bearing on the speed of train cars in a collision.

Concept Questions: Diagrams

These ask you a simple question based (obviously) on a diagram.