Two identical balls with completely smooth surfaces are moving uniformly in free space without rotation. At some moment, they undergo a perfectly elastic glancing collision. After the collision, the angle (in degrees) between the two vectors of velocities is
(d) Impossible to answer without knowledge of the angle between the two velocities before the collision.
The answer is apparently (c), but I thought that it would be 180 degrees, since the balls are glancing off each other, and clearly by drawing the plane of incidence of 1 ball one sees that the other should barely change its angle... Can someone please explain?