08 Rugbyball

08 Rugbyball#

Aim#

To show how a rotating rugby ball lifts itself

Subjects#

  • 1Q60 (Rotational Stability)

Diagram#

../../../../../_images/figure_0118.png

Fig. 273 .#

Equipment#

  • Rugby ball

Presentation#

The rugby ball lies on the floor. By hand it is given a fast spin around its short axis (see Figure 274).

../../../../../_images/figure_190.png

Fig. 274 .#

When the ball has made some turns it lifts itself, finally standing on its nose (tail) and rotating around its long axis.

Explanation#

  • When the ball turns around its short axis ( \(\omega_{s}\) ) it will tilt its long axis a little due to unbalanced mass distribution. Then spinning around its long axis \(\left(\omega_{/}\right)\) will start (see Figure 275) and at the same time, the long axis starts a precession ( \(I_{2} \omega_{1}\) moves into the direction of \(T_{p}\) ).

../../../../../_images/figure_240.png

Fig. 275 .#

  • The point of contact slips on the floor (see Diagram). The friction force \(\left(F_{f}\right)\) on the ball is pointing in the same direction as its direction of precession. The torque ( \(T_{f}\) ) of this friction force is pointing upward (see Diagram), almost perpendicular to \(I_{2} \omega_{\nu}\). So the friction force gives a torque that erects the ball ( \(I_{2} \omega /\) moves into the direction of \(T_{f}\) ).

  • See also the demonstrations Precession 3a and Precession 3b in this database where it is shown how a precessing object reacts to an applied torque.

Remarks#

  • Friction between ball and floor must be high enough to make this demonstration successful.

  • This demonstration can also be done with a hardboiled egg. Doing it on my kitchen table, the angular speed of the egg must be quite high to reach the lifting effect.

Sources#

  • Borghouts, A.N., Inleiding in de Mechanica, pag. 230-231

  • Friedrich, Artur, Handbuch der experimentellen Schulphysik, part 2, Mechanik der festen Körper, pag. 233-235

  • Roest, R., Inleiding Mechanica, pag. 230-231

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