“Mad Science” Mini-Lessons

9. Centering an Object


Physics, friction, torque, balance, center of mass


One extended rigid object per group.  Each object must be about 1 to 1.5 m long, light enough to rest on two fingers, and smooth enough to slide along a finger without catching.  Suitable objects include thin boards, meter sticks, any implement with a long handle or the handle itself (mop or mop handle, axe, hoe).


Hold your forearms parallel to the floor with your hands extended, palms facing inward.  Your hands should be at least shoulder width apart.  Rest the object on your index fingers.  Slowly bring your hands together, keeping them level, until they meet.  What happens to the object?

If you brought your hands together slowly and steadily enough, it should still be balanced on your fingers!

Now begins the real work of this activity: finding out why.  What sort of “feedback” kept the object balanced on your hands throughout the exercise?  Did it depend on any conscious action from you?  As a group and then as a class, make a thorough explanation of why the object remains balanced and centered.  Carry out additional experiments if more questions arise.

The science details

The closer a support finger is to the object’s center of bass, the more of the object’s weight it supports.  The greater the weight on a finger, the greater the force of friction to keep the object from slipping.  So the object will stick to the support finger that is closest to its center of mass.  As the fingers move together, the object’s center of mass stays between them.

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Copyright © 2004, Richard Barrans
Revised: 21 December 2016; Maintained by Richard Barrans.
URL: http://www.barransclass.com/madsci/MadSci09_Balance.html