Inertial String Snapper

Which string will break?

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Purpose -

This cool little demo shows the effects of the inertia of a mass.

Nature - 

Demonstration

Materials - 

Retort stand with boss head and clamp, cotton thread, a mass hanger with about 300 g of mass (or any other mass that you can attach a thread to top and bottom)

Method - 

· Attach one length of cotton to the top and one to the bottom of the mass hanger.

· Attach the other end of the top cotton to the retort clamp so that the mass hangs loose and the bottom thread hangs down.

· If you pull slowly on the bottom cotton you will find that the thread above the mass breaks, however if you jerk the bottom cotton quickly, the string below the mass will break.

Safety -

Nil 

Explanation -

The large mass in the centre has inertia.  If you pull slowly then the mass is working with you to help break the top cotton.  However when you jerk quickly you are working against the inertia of the mass as well as the strength of the top cotton, so the bottom cotton breaks before the mass has a chance to start moving.

Notes -

I like to ask the students which thread they think will break, then make sure that the opposite of what most of the students say is what happens.  Then I show them the other.