Suppose there are 3 balls colliding at the same time. I find that the order in which I resolve collisions makes a difference in the final result, which ofcourse makes no sense. To explain and keep things simple, consider 3 balls in 1D, all same mass, elastic collision. The numbers at the top are the speeds and the arrows is the direction. Assume they are currently all touching each others, i.e. in collision Code (Text): -->2 -->1 <---3 O O O A B C This shows ball A hitting ball B from the back and ball B and C are colliding face on. Now if we resolve collision A with B first, followed by resolving collision B with C, but using the new speed of B, this should give the same result if we instead have resolved collision of B with C, followed by resolving A with B (using the new speed of B). But it does not. first case: A with B, followed by B with C A with B gives Code (Text): -->1 -->2 O O A B and B with C gives (but using new B speed of 2 above, not the original speed of 1) Code (Text): <--3 -->2 O O B C Hence the final result is Code (Text): -->1 <--3 ---->2 O O O A B C second case: B with C, followed by A with B B with C gives Code (Text): <--3 --->1 O O B C A with B (but using new speed of B of 3 above, not original 1) Code (Text): <--3 -->2 O O A B Hence final result is Code (Text): <--3 -->2 ---->1 O O O A B C You can see the final state is different. What Am I doing wrong? and more importantly, what is the correct method to handle this? For simulation with many balls and also collision with walls, this case is very possible. (for example, ball hitting a wall and being hit by another ball at the same time, would give same problem as above, the order gives different results). Currently I use a loop to iterate over all objects and resolve collisions between each 2 at a time. Hence the order I use is arbitrary (order is just the index of the ball in an array).