# Rolling ball hits the wall

I play with a small ball in my home. I was experimenting with different ways to hit a ball along the ground and saw a somewhat strange reflection from the wall.

If I hit the ball right at the middle (it only has a forward roll), and it hits the wall at an angle, it reflects away making an equal angle from the normal to the wall to the other side as expected (of course, the collisions are not elastic and the floor is not regular, so there is some negligible error). However, if I hit the ball at its side almost tangentially (giving it a mixture of a forward spin and a side spin), it hits the wall and comes back along the same side of the normal to the wall but not necessarily making an equal angle (depending on how perfectly I hit it tangentially). Basically what I see is - I am hitting the ball on its right (and to my left) and it is coming back to me. Does this have something to do with the friction from the wall which is not present when the ball is hit with only forward spin?

• It sounds like you're talking about putting English on a billiard ball. Aug 18, 2018 at 18:36

Does this have something to do with the friction from the wall which is not present when the ball is hit with only forward spin?

Yes. When the ball with a side spin touches the wall, it is akin to a spinning wheel touching a road: in both cases, due to the friction, a spinning motion changes to the rolling motion with the associated linear velocity of its COM, parallel to the ground.

In the case you've described, this additional velocity component is subtracted from the original velocity component parallel to the wall and changes the angle of reflection.

If the relationship between the spin, speed and angle are just right, the original velocity component parallel to the wall could be exactly cancelled and the ball would be reflected right back.

That is correct. These types of friction cause all kinds of interesting phenomena in billiards and pool, see e.g.: http://billiards.colostate.edu/physics/Alciatore_pool_physics_article.pdf