Stop the spinning star

1 · Float the star

The 51 kHz standing wave is ready. You place a star with twelve curved arms in its nodal plane.

Beginner

In the well-aligned cavity, which of these particles starts spinning on its own?

A pentagon with five equal sides A star with straight arms A star with curved arms

2 · Measure the drag

To separate the push of the sound from the brake of the air, the team changes the voltage abruptly and watches the star readjust.

Beginner

Why does the team conclude that it is the push of the sound, not the drag of the air, that changes sign as the volume rises?

Drag can slow a rotation, never turn it around The drag of the air does not depend on the sound level The drag was measured to be zero at high voltages

3 · Stop the star

You hold the volume knob. Every voltage you set adds a point to your measured curve.

Intermediate

A star stands still at 10 volts. In the paper's model, which value of the coefficient α accounts for this reversal voltage?

α = −0.1 V⁻² α = −0.01 V⁻² α = −0.001 V⁻²

4 · And with three arms?

For 3- and 5-armed stars, the speed rises across the whole measured voltage range and never reverses: the fit gives them a weakly positive α, and β stays small.

Expert

According to the paper's model, what would a three-armed star do if the voltage could be raised far beyond the 15 volts tested?

It would reverse too, but at a higher voltage than the twelve-armed stars It would never reverse: its correction pushes in the same direction It would level off: the drag, rising with the sound, would eventually hold it back

Star stopped

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