Suppose that a motorboat is moving at 40 ft=s when its motor suddenly quits, and that 10 s later the boat has slowed to 20 ft=s. Assume, as in Problem 2, that the re- sistance it encounters while coasting is proportional to its velocity. How far will the boat coast in all?

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.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.

1 is a answer of a question