The sliding hand
I've been trying some practical verifications of the conservation of angular momentum. I tied a small roll of sellotape to a length of fishing line and hung it from the ceiling. Pinching the line between thumb and forefinger I started the roll moving in a circle (conical pendulum). To measure the motion I put my phone on the floor with the front camera activated so I could see what I was recording. I then recorded the motion as I moved my hand down the line. As expected the angular velocity increased but when I analysed the motion with LoggerPro the angular momentum was far from conserved. Not sure why this is but would be an interesting investigation. One factor might be that the motion of the roll is elliptical rather than circular and some energy is lost due to air resistance making the time period longer than it should be for the smaller radii. The effect of the sliding hand may also take a part.