Book Thumbnails on Physics (viscom)

I have a list of twelve words and then a few words that are on reserve.  There were a few words I couldn't get five different and good thumbnails for, so I did a few extra words and thumbnails to have just in case.  My first choice words are

gravity

buoyancy

magnetism

waves

friction

heat

acceleration

elasticity

tension

momentum

molecular

rotation

And my back up words are nuclear, mechanics, conserve, energy, and density.
These words were a bit harder to come up with different (molecular and nuclear thumbnails were similar) and good (only one good one for enery and density) thumbnails.  All twelve of my primary words generated five thumbnails but I'm curious about my "friction" thumbnails.  It's hard to literally represent friction through images let alone abstractly with just dots.