A skill I’ve always found equal parts distracting and fascinating is the ability to twirl a pen between your fingers. After countless time of attempting this trick and looking like a fool, I realize this trick is all about the physics. As a newly enlightened physics student with a better understanding of rotational motion and friction, I tried breaking down this skill into its individual concepts in an attempt to teach myself.
Axis of rotation and center of mass: The key to setting up this skill is to balance the pen at its center of mass. The location of our finger at the pens center of mass acts as its axis of rotation allowing it to be fluidly swirled. Holding the pen at either end before spinning would take even more force to set into motion because there is more weight that needs to stay balanced parallel to the ground.
Friction: The next concept I realized is helpful is the fact that frictional force of our finger keeps the pen from falling to our palm and in keep the pen in motion by supporting the rotation. As a beginner pen-twirler I went in slow motion to practice the motion, and I found that wearing a glove increased the friction, which was helpful in the beginning twirls.
Rotational acceleration: After starting from rest, the thumb and forefinger making a snapping motion and it is the force of the thumb on the pen that applies the external force necessary to increase the rotational velocity and acceleration. This sets the pen into motion after which it should theoretically be caught after one rotation, or twirled between multiple fingers creating a cool illusion.
After an embarrassing number of attempts, while I realize I still can’t twirl my pen, I at least now understand the physics behind this mesmerizing trick (which is equally as cool).