05 Coriolis (3)

Aim

To elucidate the direction of the Coriolis force on our rotating earth.

Subjects

• 1E20 (Rotating Reference Frames)

• 1E30 (Coriolis Effect)

Diagram

Fig. 55 .

Equipment

• Globe.

• Flexible straw (\(v\) and \(\omega\) in Diagram B).

• Toothpick (\(F\) in Diagram B).

Presentation

On the globe our local position is indicated by sticking a small puppet at our coordinates (Delft, \(52^{\circ}\) northern latitude; see Diagram A). On the globe the sense of rotation is indicated by arrows stuck to the equator. This sense of rotation is also indicated by the \(\omega_{0}\)-vector stuck into The North Pole.

The flexible straw is used as a resource to indicate simultaneously the direction of \(\omega_{0}\) and the direction into which an object is moving (velocity \(v\) ). The long arm of the straw is used to indicate the direction of \(\omega_{0}\) and the short arm used to indicate the direction of \(v\). Applying the corkscrew rule ( \(\vec{F}_{\text {cor }}=-2 m(\vec{\omega} \times \vec{v})\) ), the direction of \(F_{\text {cor }}\) is indicated by sticking the toothpick into the elbow of the flexible straw (see Diagram). The advantage of using the flexible straw is that easily the angle between \(\omega_{o}\) and \(v\) can be changed; the toothpick can be easily shifted in and out the elbow when the corkscrew rule indicates that the direction of \(F_{\text {cor }}\) is different (see Figure 56).

Fig. 56 .

Remarks

• On the Southern hemisphere this straw-toothpick vector model can be used when you keep \(\omega_{0}\) pointing upwards, meaning that you have to keep the model a distance away from the globe.

Sources

• Mansfield, M and O’Sullivan, C., Understanding physics, pag. 182

• McComb,W.D., Dynamics and Relativity, pag. 137-145

• Roest, R., Inleiding Mechanica, pag. 197-202; 205-210

• Giancoli, D.G., Physics for scientists and engineers with modern physics, pag. 291-292