How can an object that's not accelerating cause another stationary object to move in a collision? This website: http://www.physicsclassroom.com/Class/momentum/U4l2a.cfm, says that a "rightward moving" 7-ball experiences a leftward force when it hits an 8-ball, and the 8-ball experiences a rightward force, but where is this force coming from when the ball was moving with a constant velocity before the collision?
How does Newton's third law explain any collisions that involve a constant velocity?
EDIT: Does it slow down because of the normal force? And then, is there a normal force action-reaction pair? And, if so, how do you calculate the magnitude of the normal force?