The Cheerios Question

A Physics students makes an observation one morning and then asks several friends, what happens when two cheerios are placed near each other in a bowl of milk (and why).

• A. The first student says - nothing particular happens. The Cheerios drift around at random (and become soggy). There are no net forces on the cheerios, so they just move with any random currents in the milk..
• B. The second student says that the Cheerios actually move apart, assuming the milk is at rest. They move apart because no matter how gently they are placed in the milk, tiny waves are generated by each Cheerio. These waves strike the other Cheerio causing it to move away from the first Cheerio.
• C. The third student says that the Cheerios actually move towards each other. This is caused by capillary action. The milk rises on the sides of the cheerios causing a horizontal component of force, which causes the cheerios to move towards each other.?

See if you picked correctly.

Answer

The Cheerios Question - Answer = C

Answer C is correct. The Cheerios come together. Capillary actions causes the milk to move up as shown in the Diagram. There is a capillary force lifting the water due to the Cheerio. There is an equal and opposite force on the Cheerio due to the water, which has two components, as shown. The horizontal component of this force causes the Cheerios to come together. There is of course capillary action all around the Cheerios, but the nearness of the two cheerios cause the capillary effect to be stronger on the sides of the cheerios next to each other. (Some information from The Flying Circus of Physics by Jearl Walker)

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