Note:  This is the first in a series of posts about science activities to be done in the bathroom – the lav-oratory if you will.  This is an easy experiment to get you started.  The messier ones come later.

Before we sit down on the toilet we need to define inertia.  The concept comes from Newton’s first law that most middle schoolers can recite “An object at rest remains at rest and an object in motion remains in motion (in a straight line) unless acted on by a force”.  Inertia is simply this tendency to remain at rest or motion and depends on the mass (or weight) of the object.  In our case, the object is a roll of toilet paper.

This demonstration works best with a full roll of toilet paper, hanging on a roll like the picture here.  (It does not matter if the end is oriented on top or underneath – we won’t debate the advantages of each here).

First, pull gently on the roll so the toilet paper starts to unroll.  You have an object in motion!  As long as you are pulling, the roll keeps on rolling.  And if  you get enough toilet paper on the floor it may keep on rolling on its own.

Now hold on to the end of the toilet paper (near the roll) and give it a quick yank.  The paper in your hand should rip right off.  In this case, the inertia of your “massive” toilet paper roll kept it at rest while the less massive piece ripped right off.

Does this always work?  Here are some ideas for experimenting:

  • How much toilet paper needs to be on there fill for this to work?  Will it work with half of a roll?  a quarter of a roll?
  • Does this work with paper towels too?
  • Do the perforations matter?  Can you control if the paper rips at a perforation or in the middle of a piece?
  • How else do you use inertia in the bathroom?

Learn more about toilet paper in the Toilet Paper Encyclopedia.
And check out this video of the great physics teacher, Paul Hewitt, explaining this same concept.

May your life be long and useful like a roll of toilet paper”  Anonymous Proverb

Eureka! Toilet Paper Inertia
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