Okay i'm totally stuck and nobody i know really gets it either, so i've turned to yahoo for : )
basically there are four aliens on a planet. they want to go on a sea-saw. the end seats on the sea-saw are 2m from the pivot, and the middle seats are 1m from the pivot. the aliens way 400n, 300n, 200n and 100n - which two can sit on the sea-saw (on any of the seats) and make it balance? it'd be a huge if you at least me work out the answer! : )

a longitudinal wave moves energy that is to the direction of the wave’s movement. - true

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.