A yo-yo has, as we all know, a centre axle. Pulling the string means pulling a bit away from the centre. That creates a torque, and so it rotates.
Now, pull the string of a yo-yo lying on a table. Which way will it roll?
- With the string on the upper-side, it moves forwards along with the pull. That’s obvious, since your pull creates a torque that causes it to roll forward (clockwise). Torque and pull both make it move forwards.
- But with the string on the under-side, which way will it now move? The pull is forward, but it creates a torque that causes a backwards (counterclockwise) rotation. Which of these two will prevail?