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bobey
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(Moderator's note: thread moved from "Differential Equations")
A particle of mass m which moves along a horizontal straight line with a velocity of v encounters a resistance of av + b(v^3), where a and b are constants. If there is no other force beside the resistance acting on the particle and its initial velocity is U , show that the particle will stop after it has moved a distance of m(ab)^(-1/2)tan^(-1)(U(b/a)^1/2). Also show that the velocity is 1/2U after time (m/2a)ln(4a+b(U^2))/(a+b(U^2)))
to show the distance when the particle stops :
i use :
F = ma
=> m (dv/dt) = - (av+b(v^3))
i let v = y and t = x to make it less confusion ...(1)
thus m(dy/dx) = -(ay+b(y^3))
dy/dx = -(ay/m)-(b(y^3)/m)
dy/dx + (a/m)y = -(b/m)(y^2) ... (2)
(y^-3)(dy/dx) + (a/m)(y^-2) = (-b/m) ===> this is Bernoulli equation
let v = (y^-2)
thus, dv/dx = -2(y^-3) (dy/dx)
=> -1/2(dv/dx) + (a/m)v= -b/m
searching for integrating factor, miu(x) = exp(integrating(a/m)dx)
thus, miu (x) = exp(ax/m)
miu(x) x (2) : d/dx (exp(ax/m).v) = exp(ax/m).(-b/m)
exp(ax/m).v = (-b/m) integrating (exp(ax/m) dx)
v = (-b/a) + c(exp-(ax/m))
but v = (y^-2) = (v^-2)
thus,
v^-2 = (-b/a) + c(exp-(ax/m))
v^2 = (1/c(exp-(ax/m))) - (a/b)
v = sqrt ((1/c(exp-(ax/m))) - (a/b))
applying initial condition where v_0 = u, x=0, t=0
c = 1/u^2 + b/a
thus v = dx/dt = sqrt ((1/(1/u^2 + b/a)) - a/b)
when particle stop dx/dt = 0
then
0 = sqrt ((1/(1/u^2 + b/a)) - a/b)
.
.
.
x = m/a ln |(1/ab)+1|
why i can't get the answer given which is m(ab)^(-1/2)tan^(-1)(U(b/a)^1/2)??
where I'm going wrong?
please help me!
p/s : i don't know to use SYNTAX. sorry =p
A particle of mass m which moves along a horizontal straight line with a velocity of v encounters a resistance of av + b(v^3), where a and b are constants. If there is no other force beside the resistance acting on the particle and its initial velocity is U , show that the particle will stop after it has moved a distance of m(ab)^(-1/2)tan^(-1)(U(b/a)^1/2). Also show that the velocity is 1/2U after time (m/2a)ln(4a+b(U^2))/(a+b(U^2)))
to show the distance when the particle stops :
i use :
F = ma
=> m (dv/dt) = - (av+b(v^3))
i let v = y and t = x to make it less confusion ...(1)
thus m(dy/dx) = -(ay+b(y^3))
dy/dx = -(ay/m)-(b(y^3)/m)
dy/dx + (a/m)y = -(b/m)(y^2) ... (2)
(y^-3)(dy/dx) + (a/m)(y^-2) = (-b/m) ===> this is Bernoulli equation
let v = (y^-2)
thus, dv/dx = -2(y^-3) (dy/dx)
=> -1/2(dv/dx) + (a/m)v= -b/m
searching for integrating factor, miu(x) = exp(integrating(a/m)dx)
thus, miu (x) = exp(ax/m)
miu(x) x (2) : d/dx (exp(ax/m).v) = exp(ax/m).(-b/m)
exp(ax/m).v = (-b/m) integrating (exp(ax/m) dx)
v = (-b/a) + c(exp-(ax/m))
but v = (y^-2) = (v^-2)
thus,
v^-2 = (-b/a) + c(exp-(ax/m))
v^2 = (1/c(exp-(ax/m))) - (a/b)
v = sqrt ((1/c(exp-(ax/m))) - (a/b))
applying initial condition where v_0 = u, x=0, t=0
c = 1/u^2 + b/a
thus v = dx/dt = sqrt ((1/(1/u^2 + b/a)) - a/b)
when particle stop dx/dt = 0
then
0 = sqrt ((1/(1/u^2 + b/a)) - a/b)
.
.
.
x = m/a ln |(1/ab)+1|
why i can't get the answer given which is m(ab)^(-1/2)tan^(-1)(U(b/a)^1/2)??
where I'm going wrong?
please help me!
p/s : i don't know to use SYNTAX. sorry =p
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