PURPOSE: To demonstrate the Magnus effect.

DESCRIPTION: A rough-surfaced drum rotates rapidly counterclockwise as viewed from above. When the airstream from a fan is blown past the rotating drum from the rear, as shown in the photograph, the cart moves along the track to the left, due to the Magnus effect.

According to the Magnus effect, air gets caught up by the surface of the rotor, and follows the motion of the rotor to the lagging side (right side of rotor in photograph), where it is ejected as vortexes shedding to the right. The reaction force on the rotor causes it to move to the left, as seen in a short video by using your mouse to select the mpeg or movie format.

Note that it is incorrect to use the Bernoulli effect in explaining this demonstration. The effect is due to shedding of vortexes, NOT to squeezing of flow lines that create a lower pressure on one side of the cylinder compared with the other.

SUGGESTIONS:

See Question of the Week #220 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 2C20.80) See slide of Flettner ship in our slide file, and film loop MAGNUS EFFECT; see Demonstration Reference File for further information. See also the web description of a non-rotating mast ship.

EQUIPMENT: Cart with spinning drum, track, and fan.

SETUP TIME: 5 min.