Moji listed the solids in order of hardness as graphite < chalk < quartz. His peer group suggested some experimental errors that may have caused this to
happen. Which experimental error most likely led Moji to this conclusion?
A. Moji might have used color as an indication of hardness.
B. Moji might have thought that crumbliness was a sign of softness.
C. Moji might have thought that crumbliness was a sign of hardness.
D. Moji could have chosen hardness tests that were not reliable.

D

Explanation:

Moji could've chosen hardness tests that were not reliable.

russian is not my first language, so i'm afraid i can't write out an answer in russian, but hopefully you can find a way to do so. here's my best translation of the problem: you're given a plot of a particle's position in one-dimensional space over time, and you're asked to find either the total distance traveled by the particle after 6 seconds had elapsed, or the total displacement of the particle after 6 seconds.

if you want the total distance traveled, you would first need to determine the particle's velocity as a function of time, then integrate that over the 6-second interval. we know the position function is quadratic with roots at 0 and 6, so we have

where is unknown. because the position is parabolic, there is symmetry about the midpoint at , and we know that

so that the position function is

which means the velocity is

then the total distance traveled is

so the total distance traveled is 20 m.

on the other hand, if you want to find displacement: the particle starts at and ends at , so its displacement is 0.