Animating a circle inside a rectangle

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David Eufrásio 2020-3-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/512247-animating-a-circle-inside-a-rectangle

评论： David Eufrásio 2020-3-28

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.

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采纳的回答

darova 2020-3-22

2
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/512247-animating-a-circle-inside-a-rectangle#answer_421364

在 MATLAB Online 中打开

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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David Eufrásio 2020-3-22

在 MATLAB Online 中打开

Thank you so much for your help.

Though, I tried what you said, but there are 3 questions that popped up:

Whenever I change the velocity, the less positions the circle has... Shouldn't velocity not have any effect on that?
The circle shows 'dragging' (not only 1 circle appears). Could that be a result of the 'hold on' // 'hold off'? Also, there is a circle on (0,0), I can't find the reason why...
While running the code, the circle stops after getting to x=14 , y=9 , and it doesn't return - shouldn't the sign change on the equation?

Thank you once more. Here's how the code is currently:

a=20
b=10
R=0.5
v=[10;10]
r=[10;5]
vx=v(1)
vy=v(2)
x=r(1)
y=r(2)
dt=0.1
deltat=5
X=[x zeros(1,deltat)]
Y=[y zeros(1,deltat)]
%x to the right
xdireita=((a-R-x))/vx
%rectangle
plot([0,a],[0,0],'r-',[a,a],[0,b],'r-',[0,0],[0,b],'r-',[0,a],[b,b],'r-')
hold on
for i=2:1:deltat
X(i) = X(i-1) + vx*dt
Y(i) = Y(i-1) + vy*dt
if (X(i)<0)||(a<(X(i))
    vx = -vx;          
end
if (Y(i)<0)||(b<Y(i))
    vy = -vy;           
end
plot(X, Y,'ro','MarkerSize',28*R);
axis ([-10,30,-10,20])
pause(0.05)
end
hold off

darova 2020-3-28

Why can't you declare deltat inside function?

David Eufrásio 2020-3-28

Basically, some steps ahead in the main script the deltat (the time a particle takes to collide with a wall, or with another particle) is calculated for each cycle the code does, and the animation will draw the movement for that deltat. The problem is that sometimes deltat isn't an integer number (it's decimal), and therefore the instruction zeros gives an error.

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