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.