# Collision detection between a moving circle and stationary point

1. Aug 15, 2010

### ZippyDee

I'm trying to figure out the best way to find a collision between a moving circle and a stationary line segment. I have the x and y of the point, and the x, y, radius, x velocity, and y velocity of the circle.

//Edit:
I tried using the equation of a circle with the origin at the point (P) and a radius equal to that of the original circle, and then solving that for x, substituting the equation for the line of motion of of the ball in place of y:

m=(ball.yVelocity)/(ball.xVelocity)
b=ball.y-ball.x*m
r2=((mx+b)-Py)2+(x-Px)2

I multiplied it all out and simplified it to standard form. Then I collected like terms and came up with these variables

A=m2+1;
B=2*m*b-2*m*Py-2*Px;
C=b2-2*b*Py+Py2+Px2;

Then I used the quadratic formula to find the intercepts

That should work, but it doesn't...

Last edited: Aug 15, 2010
2. Aug 15, 2010

### ZippyDee

Wow...never mind, I am an idiot...I forgot to subtract r2 before collecting terms!

3. Aug 15, 2010

### ZippyDee

Oh...but this doesn't work if the circle has an x velocity of 0...How do I deal with that?