How can i make my ODE graph make sense?

Question

Ian Samuelsson 2021-10-18

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https://ww2.mathworks.cn/matlabcentral/answers/1566551-how-can-i-make-my-ode-graph-make-sense

评论： Star Strider 2021-10-18

I don't know why my graph is like that ! I tried to plot the differential and the solved ODE but my graph dont make sense.

I'm trying to make a graph of a ODE about a ball dropping in oil, the values are all random, just to plot the thing

clear; clc;

clearvars;

syms t v(t) aa g p p1

aa = 5;

u = 3;

g = 10

p1 = 9;

p = 3;

v0= 0;

cond = v(0) == v0

eqn = diff(v,t) == (1-p1/p)*g-(9*u/2*aa^2*p)*v

vSol(t)= simplify( dsolve(eqn,cond) )

fplot(t,vSol(t),[0 10000]); hold on; grid on;

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Answer 1

Star Strider 2021-10-18

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1566551-how-can-i-make-my-ode-graph-make-sense#answer_811161

在 MATLAB Online 中打开

The fplot call is incorrect, and the function declines so quickly that it’s not possible to see it clearly with a time interval going from 0 to 1E+5.

Correct those and it works!

syms t v(t) aa g p p1

aa = 5;

u = 3;

g = 10

p1 = 9;

p = 3;

v0= 0;

cond = v(0) == v0

cond =

eqn = diff(v,t) == (1-p1/p)*g-(9*u/2*aa^2*p)*v

eqn(t) =

vSol(t)= simplify( dsolve(eqn,cond) )

vSol(t) =

figure

fplot(vSol(t),[0 1E-2]); hold on; grid on;

.

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Star Strider 2021-10-18

在 MATLAB Online 中打开

No worries!

The quiver function wants numerical values, not symbolic values, and those are easily provided.

I’m not certain what the desired result is, however I created some results that might be close to it. If they’re not, please describe in detail what the desired result is, and I’ll do my best to help provide it.

syms t v(t) aa g p p1

%alpha = r

aa = 5;

%u=mi

u = 3;

%gravity

g = 10

%density'

p1 = 9;

%density

p = 3;

v0= 0;

cond = v(0) == v0

cond =

eqn = diff(v,t) == (1-p1/p)*g-(9*u/2*aa^2*p)*v

eqn(t) =

vSol(t)= simplify( dsolve(eqn,cond) )

vSol(t) =

fplot(vSol(t),[0 1E-2]); hold on; grid on;

Ax = gca;

% get(Ax)

T = Ax.Children.XData; % Get Data From Plot

V = Ax.Children.YData; % Get Data From Plot

ang = atan2(V,T); % Use Four-Quadrant Arctangent

% xx = linspace (-5, 5 , 30); yy=xx;

% [V,T]= meshgrid(xx,yy);

% Uq= cos(ang);

% Vq= sin(ang);

Uq = gradient(T); % Numerical Derivative

Vq = gradient(V); % Numerical Derivative

quiver(T,V,Uq,Vq); hold on; grid on;

%quiver(t, v(t), [0 10 100]);

Experimment to get the desired result.

.

Ian Samuelsson 2021-10-18

is a ODE :

[A body falling in a relatively dense fluid, oil for example, is acted on by three forces (see Figure 2.3.5): a resistive force R, a buoyant force B, and its weight w due to gravity. The buoyant force is equal to the weight of the fluid displaced by the object. For a slowly moving spherical body of radius a, the resistive force is given by Stokes’s law, R = 6πµa|v|, where v is the velocity of the body, and µ is the coefficient of viscosity of the surrounding fluid...]

i finish everything, i'm just suferying in the graphs ahahahah.

@Star Strider About the disable is just turn off the hold ? putting hold off ? i did not understand that part.