Monday, October 13, 2014

The Pop Can Race

My 7 year old has been checking out science experiment books from her school's library which has given us several ideas for testing out different concepts in science.  Recently, one science experiment struck me as odd, and well...simply wrong!  The idea of the experiment is very simple.  Take a full pop can and an empty pop can of the same size, start them at the top of a ramp, and release at the same time.  Which reaches the bottom first?

Galileo showed that all objects accelerate at the same rate regardless of size/shape/mass, in the absence of friction.  Without air, a golf ball and a feather, when dropped at the same time will reach the ground at the same time.  This has been shown in vacuum tubes when the air is pumped out.  In real life situations, friction always plays a role.  The science experiment book stated that the empty pop can will always reach the bottom of a ramp first because there's less friction acting on it.

This immediately struck me as incorrect because friction isn't the only force in play.  Granted, the force of friction between the can and the ramp is probably greater for the full can.  I'm not denying that but air friction probably plays a bigger role on the lighter can.  I don't know for sure, I'm just suspecting this.  However, my concern here isn't friction.  My concern is the rotation of the can.  Rotating objects are a bit more complicated.  If the mass is distributed in different ways, the inertia of the object differs.  An empty can, with all of the mass on the outside, has a greater moment of inertia than the full can.  As a result, the empty can takes a bit longer to start its motion.

This is a common physics topic in my high school classes and there are several demos I show with rings and solid cylinders to hammer this point home.  The solid cylinders always beat the rings in a race down the ramp.

My daughters and I decided to test this out ourselves and sure enough, the full pop can ALWAYS reaches the bottom of the ramp first.  The book is simply wrong on this.  See our video below.

video

Later we raised and lowered the ramp and the full pop can still ALWAYS wins!  As you can tell from the video, my 4 year old was quite excited!  LOL!  Try it yourself.  If your kids get as excited as mine were in the video, then you've killed a good 30 minutes of time on a rainy or too hot day!

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