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Q & A: A climbing brush

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Most recent answer: 05/16/2013
I hold a paper towel cardboard tube vertically into which I insert the top of a bottle cleaning brush from the bottom. I gently pat the top of the tube and the brush keeps moving up into the tube. Can you please explain the physics behind this?? Thanks in advance.
- gokul (age 10)
lansing, michigan, usa
It sounds to me like a ratchet effect.   When you tap the top of the tube it jerks downward.
The brush doesn\'t move quite as far because it has a bit of inertia.  Now when the tube stops moving the hairs of the brush keep the brush from falling back down, hence, it has moved a small bit towards the top of the tube.    Neat trick.

(published on 05/16/2013)

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