this illustration is about a motor boat in

a lake. here we are given that a motor boat of mass m, moves in a lake with velocity v

not. at t equal to zero the engine of boat is switched off. and the water resistance

is proportional to speed of boat and, it varies as f r is minus r v. where r is a constant.

and we are required to find the time for which boat will move before coming to rest. and

also we are required to find the average velocity of boat during the interval its velocity changes

from v not to v not by 2. so here. first we can directly write, that the acceleration

of boat. can be written as ay is equal to minus r by m, multiplied by v. because in

this situation we are given that resistance force is minus r v. so in this situation we

can write ay as d v by d t so. d v by d t is minus r by m, times v. which can be written

as d v by v is equal to minus r by, m d t. so if we integrate this. term, at t equal

to zero the speed was v not at, time t. the speed changes to v. so lefthand side becomes

ellen of v. i am substituting limits this will be ellen of v by v not. is equal to minus

r t by, m. on simplifying this will give us the speed as v not e to power minus, r t by

m. so here we can see the value of v is equal to zero. when, here t is tending to infinity,

when this t tends to infinity then only the speed. become zero. and here we are required

to find the time for which boat will move before coming to rest. so v equal to zero

which signifies rest. so boat will come to rest only when, t tending to infinity after

very very long time this will be the result of the first part of the problem. and here

we are also required to find, the average velocity of boat during time interval its

velocity changes from v not to v not by 2. so if we continue, here. we can write, at

t equal to zero the value of speed was v not. and we can write at t is equal to t 1. if

speed changes to v not by 2. then using this expression, we can find out the time t 1.

so if we write this as equation 1 then we can write from 1. we can substitute v as v

not by 2 so this v not by 2 is equal to. v not e to power minus r t by, m. and the value

of t we can put as t 1. so in this situation this v not gets cancelled out and on lefthand

side it can be written as, ellen 2 is equal to r t 1 by m this implies the value of time.

after which, the speed reduces to v not by 2. is, m by, r. ellen 2. so now, we can directly

calculate the average velocity by the average function which is given as 1 by t 1 integration

from zero to t 1. this v as a function of time d t. on substituting the value this is

1 by t 1 integration zero to t 1. and v as a function of time as v not e to power minus

r t by, m. d t. so if we integrate this term, the average velocity here can be given as,

now on integrating this term it is v not by t 1 and if we integrate e to power minus r

t by m. this will be v not m by, r t 1. and its integration remain same this is minus

e to power, minus r t by, m. and limits from zero to t 1. so if we substitute the value

of t 1 it is m by r ellen 2. so the lefthand side here it becomes, v not, by ellen 2. and

this term if we substitute t 1 this will become. minus e to power minus r by m t 1 which can

be written as minus ellen 2. and if we substitute zero this will become plus 1 so we can write

it 1 minus, e to power minus ellen 2. and, this can be further written as e to power

minus ellen 2 it can be written as 1 by 2, so the result is v not by 2 ellen 2, that

will be the final result of the problem.

Good Illustration ..

sir we can also do it by finding- total distance/total time

if we use a=vdv/dx

sir can we use

{average velocity=total displacement/total time}

formula here

Why Vavg not equal to total disp. /Total time here.

The integration part was quite tricky