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I conducted a physics experiment today. I simply held up a small ball against the wall, and let go. I was expecting the ball to exhibit some rolling motion of some sort due to friction, but to my surprise, it just fell straight down. In addition, I noticed that the ball had deviated from its original straight path. Why is this so?

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  • $\begingroup$ I would like to know what kind of ball you used and with what method you let the ball drop from height. $\endgroup$ – wcc Oct 13 '18 at 2:08
  • $\begingroup$ I used those kind of small stress ball roughly the size of a fist. I let it gently touch the wall, and I let go. $\endgroup$ – QuIcKmAtHs Oct 13 '18 at 2:09
  • $\begingroup$ Stress balls are compressible so it is possible that you do a gentle press on it while letting it touch the wall. $\endgroup$ – wcc Oct 13 '18 at 2:11
  • $\begingroup$ I was actually careful not to do that $\endgroup$ – QuIcKmAtHs Oct 13 '18 at 2:28
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For the ball to remain in contact the wall, it must have either a force pressing it against the wall or be just touching the wall. In the first case, the applied force is countered by a normal force, which in turn causes friction that applies torque to cause rolling. In the second case, the ball is just touching the wall, there is no normal force, and hence no friction to cause rolling.

The first case can be demonstrated by a painter painting the wall with a roller brush. However, for the second case, you realize that it isn't possible as there is no force keeping it against the wall. The moment you release the ball, it will gain a (small) impulse in the direction normal to the wall (because by letting go, you have stopped applying the force keeping it there). The normal force will then act for a brief moment, giving that small impulse. Another way to understand this is to model the ball and wall as (very rigid) springs.

In short, this can't happen because there has to be a force pressing the ball against the wall to cause torque.

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