MATLAB: Animating a circle inside a rectangle

animationcirclecollisionMATLABrectangle

The goal is to create an animation that represents an animated (separate successive images separated by a time 'dt') particle (28*R pt, 'R' is the radius, and the position of its center is r=[x;y]) during a time interval 'deltat' within a rectangle (width=a and height=b), with a certain velocity v=[vx;vy]. This particle collides with the 'walls' of the rectangle and conserves its kinetic energy. The collision with the wall x=0 happens in a deltat=(R-r(1))/v(1), and the collision with the wall x=a happens in a deltat=(a-R-r(1))/v(1).
I tried to represent this, but there's always something wrong with the code.
[MY CODE]
/////////////////////////////////////////
a=20
b=10
R=0.5
v=[10;10]
r=[10;5]
vx=v(1)
vy=v(2)
x0=r(1)
y0=r(2)
dt=0.1
deltat=5
X=zeros(1,500)
%x to the right
xright=((a-R-x))/vx
x0 = a/2;
y = b/2;
for i=2:1:deltat
%rectangle
plot([0,a],[0,0],'r-',[a,a],[0,b],'r-',[0,0],[0,b],'r-',[0,a],[b,b],'r-')
hold on
%circle
X(i) = x(i-1) + vx*dt
plot(X, y,'ro','MarkerSize',28*R);
axis ([-10,30,-10,20])
hold off
pause(0.05)
end
/////////////////////////////////////////
My logic was to literally trace the circle's movement (first from the center to the right wall, then to the center again towards the left wall (…) ). Nonetheless, the code does not work.
Thank you.

Best Answer

Mistake:
Mistake
Also you shoud have conditions when ball collides:
if (x<0)||(a<x)
    vx = -vx;           % change sign
end
the same for vy
Add line (similar to yours)
Y(i) = Y(i-1) + vy*dt;
You can also add gravity
vy = vy - 9.8*dt;
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