Creating a Function to Plot Projectile with Drag
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Hello, I've recently been tasked with creating a program for a course regarding plotting projectile motion with drag (air resistance). The problem i have run into is regarding overwriting of variable (Vx and Vy) but whenever i replace them and put them back into the function so they don't overwrite themselves the program either does nothing or crashes matlab. since i don't have matlab at home i've been using freelab and convert back and forth as i work at uni and home.
The process i followed can be found here http://wps.aw.com/wps/media/objects/877/898586/topics/topic01.pdf
Another problem i have had is that i am unable to plot the y axis for the function without drag.
clear
close all
disp('Welcome to the Projectile Motion Plotter');
disp('This projects the motion for a tennis ball with and without air resistance');
Vx = input('Please input the horizontal velocity [m/s]: ');
Vy = input('Please input the vertical velocity [m/s]: ');
%Sets up intial conditions
V = sqrt(Vx^2 + Vy^2); %Determines V by combining v of both axes
G = 9.80665; %m/s^2 Acceleration due to Gravity
DC = 0.8; %Drag coefficient
Area = 0.0033; %m^2 cross section area of a tennis ball
Mass = 0.057; %Kg mass of tennis ball
x(1) = 0; %intial x postion
y(1) = 0; %inital y postion
xf(1) = 0; %inital xf postion
yf(1) = 0; %intial yf postion
AP = 1.2; %kg/m^3 Air Density @ Sea Level
D = AP*DC*Area/2; %constant needed for drag calculations created
t(1) = 0; %sets intial time
dt = 0.01; %s set the intervals at which time will be evalutated
i = 1; %sets counter/index
%Starts a loop for Projectile Motion with Drag
while min(y)> -0.01;
t = t + dt;
i = i + 1;
xf(i) = xf(i-1)+ Vx.*dt;
AxD = - ( D / Mass ) * V * Vx;
AyD = -G - ( D / Mass ) * V * Vy;
Vx = Vx + AxD * dt;
Vy = Vy + AyD * dt;
x(i) = x(i-1) + Vx * dt + 0.5 * AxD * dt^2;
y(i) = y(i-1) + Vy * dt + 0.5 * AyD * dt^2;
end;
plot(x,y,'b'), hold on; %plots the Projectile Motion with Drag
plot(xf,y,'r'), hold off; %plots the Projectile Motion without Drag
xlabel('Horizontal Distance (m)'); %labels the x axis "Horizontal Distance (m)"
ylabel('Vertical Distance (m)'); %Labels the y axis "Vertical Distance (m)"
title('Projectile Motion Paths'); %Gives a Title "Projectile Motion Paths"
so this is the latest and furthermost i have got, this was done in freeMat 4.2 so some small changes need to be done to make it work in matlab. Other things i have yet to do is add the y axis for projectile motion without drag and a legend for the graph. if you spot anything odd or can tell me how to stop getting those variables overwriting themselves i would be very thankful.
Bryce
6 个评论
José-Luis
2014-6-5
A possible solution: create two differently named variables
V_old, V_new
and replace where it corresponds.
Sara
2014-6-5
what do you mean that Vx is overwritten? Do you mean that:
Vx = Vx + AxD * dt;
should use Vx inputted by the user on the right side?
Bryce
2014-6-5
José-Luis
2014-6-5
Please try using the debugger.
dbstop if error
your_script.m
And look at the variables in your workspace. It would be much more efficient than us trying to debug remotely.
Bryce
2014-6-5
Douglas Leaffer
2017-11-25
Substitute V for Vx in this loop: xf(i) = xf(i-1)+ Vx.*dt;
回答（1 个）
Sara
2014-6-5
You can replace your while loop with:
while min(y)> -0.01;
t = t + dt;
i = i + 1;
xf(i) = xf(i-1)+ Vx.*dt;
AxD = - ( D / Mass ) * V * Vx;
AyD = -G - ( D / Mass ) * V * Vy;
Vx_new = Vx + AxD * dt;
Vy_new = Vy + AyD * dt;
x(i) = x(i-1) + Vx_new * dt + 0.5 * AxD * dt^2;
y(i) = y(i-1) + Vy_new * dt + 0.5 * AyD * dt^2;
Vx = Vx_new;
Vy = Vy_new;
end;
It plots something without errors.
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2017-11-25
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