# Calculate new velocity of a ball when colliding with a circle in 2D

I'm currently making a 2D game in C++ but i've come across a problem while doing collisions. I have a ball free falling

The above example shows what's going on in my game, I can detect when the two circle collide fine but I don't know how to calculate a new velocity for the ball to get it to go "around" the circle and not try to force it's way through it. Any idea on how to calculate this vector ?

Edit: forgot to mention, circle collision is static while ball is moving

• Why would the ball go "around" the circle? If I dropped a ball and it hit another ball that was fixed in place, I'd expect my ball either to go splat or bounce upward, depending on what the two balls were made of. I wouldn't expect it to simply "go around" the other ball unless the two balls had some very unusual properties. I suppose if you have a particular path and speed in mind, someone could come up with a formula for the velocity whether it makes physical sense or not. – David K Jan 2 at 17:03
• @Matt You can have the new velocity vector tangential to the black circle at the point of contact – Shubham Johri Jan 2 at 17:36
• More info is needed. Is this a collision between two moving balls? Is the collision completely elastic? What are their velocities and masses? Etc. – Jens Jan 2 at 20:31
• @DavidK You are right, the ball would bounce but before implementing it i'd like collisions to work that way, for my personal learning mainly. – Matt Jan 2 at 20:40
• @ShubhamJohri Yes, sounds like a good idea! I will take a look at that thanks – Matt Jan 2 at 20:41