As friends frequently do, Josh and John stand at either end of a very long, stationary skateboard, with Josh at the front and John at the rear. John then tosses a heavy ball toward Josh who, caught unawares, ducks so that the ball goes sailing over his head and lands on the ground. There is no friction between the skateboard and the ground. After the ball is thrown, which way, if at all, does the skateboard move

A. Backward
B. Forward
C. Neither

B. Forward

Explanation:

The net force in the system is initially balanced and the system is in equilibrium.

1) As John ducks, two things happens. First, there is a sudden impulse from his end which will change the momentum of the system from zero to an amount Mv proportional to the impulse Ft, which will be directed away from his direction because,

2) the process of ducking pushes his center of mass forward, which also moves the center of mass of the whole system away from him. This two effect will cause the skateboard to move forward.

because growing process had stoped after 4-5 year

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.