1. The problem statement, all variables and given/known data Two identical small balls are shown in picture above. A ball is thrown from the ground and another ball is dropped without any initial velocity. First, consider no air friction. In order to make the balls collide each other, the ball on ground has to make an angle α from the ground, where tan(α)=h/d. Then consider a more real case, there's air friction which is proportional to ball's velocity, F=-kv. In order to make the balls collide, determine whether angle of the ball has to be greater than α, less than α, or equals to α! (Problem's source and image: http://collectionofphysicsproblems.blogspot.com" [Broken]) 2. Relevant equations Ffriction = -kv Fgravity = mg 3. The attempt at a solution I think it has to be greater than α, is it correct?