# Matlab bouncing ball in box

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Dakota Nielsen on 27 May 2021
Edited: Jan on 27 May 2021
Hey, guys I'm new to matlab and I'm working on making a script that will create a 20x20 box and have a ball bounce continously off the walls. The program will ask the user to input the magnitude of the velocity and the angle between the velocity vector and the x-axis. It will then calculate and plot the position of the ball in a graph window.
So far this is what I have. I have the ball bouncing off the right side correctly and the top as well. But the ball isn't traveling far enough to the left and down as well. Here is the code.
set(gca, 'XLim',[-10 10], 'YLim', [-10 10]);
cla
axis square
ball = animatedline('color' , 'r', 'Marker', 'o' , 'MarkerSize',12,'MarkerFaceColor','r');
hx0 = .05
hy0 = .05 / sqrt(2);
hx = hx0;
hy = hy0;
xl = 2 %.02
xr = 9.65
yb = xl;
yt = xr;
x = 1
y = 1
while 1 == 1
if x < xl
hx = hx0
end
if x > xr
hx = -hx0;
end
if y < yb
hy = hy0;
end
if y > yt
hy = -hy0;
end
x = x + hx;
y = y + hy;
clearpoints(ball);
drawnow;
pause(.01)
end

Geoff Hayes on 27 May 2021
Edited: Geoff Hayes on 27 May 2021
Dakota - your axes limits are set as
set(gca, 'XLim',[-10 10], 'YLim', [-10 10]);
but the limits you set in code for the box are different
xl = 2 %.02
xr = 9.65
yb = xl;
yt = xr;
I think that you can either reset the axes limits once you set the above four variables with
set(gca, 'XLim',[xl xr], 'YLim', [yb yt]);
or just set your limits to that of the axes
xl = -10;
xr = 10;
yb = xl;
yt = xr;
Dakota Nielsen on 27 May 2021
Thank you so much!

Jan on 27 May 2021
Edited: Jan on 27 May 2021
Your point starts at [1,1], but it changes the direction at this strange limits:
xl = 2; %.02
xr = 9.65;
yb = xl;
yt = xr;
A simplified version with adjusted limits:
axes('XLim',[-10 10], 'YLim', [-10 10], 'NextPlot', 'add');
axis square
hx = 0.05;
hy = 0.05 / sqrt(2);
xl = -9.65;
xr = 9.65;
yb = xl;
yt = xr;
x = 1;
y = 1;
for k = 1:5000 % while 1 == 1
if x < xl || x > xr
hx = -hx;
end
if y < yb || y > yt
hy = -hy;
end
x = x + hx;
y = y + hy;
plot(x,y, '.');
end

Scott MacKenzie on 27 May 2021
There are alternatives to animatedline. You can create a ball using rectangle with the curvature property set to 1. But, given the approach you are taking and the issue in your question -- the ball isn't traveling far enough to the left and down as well -- change
xl = 2;
to
xl = -10; % adjust as per marker width