Video: Watch this demo
Description: A counter intuitive demo to teach students about torque and forces.Most people imagine the pivot point as being the center of the spool, so they imagine that pulling from the bottom will cause a torque that will make the spool roll away. In reality, the pivot is the point of contact between the spool and the table. Pulling from the bottom thus creates a net torque that still causes the spool to roll toward you.
OPTIONAL: By pulling the thread upward, you can actually make the spool roll away, as intuition would expect. There is a critical angle where this behavior occurs, and that angle should correspond to a line from the point of contact of the spool and the table, running tangent to the inner radius of the spool. This line is where your pulling would create NO TORQUE about the contact point. Moving just below or just above this angle will create a net torque counter-clockwise or clockwise about the point of contact, causing the spool to roll toward you or away from you.
- Wooden, metal, or plastic spool
- String or paper, to wrap around spool
- Make sure spool and thread are in working order.
- First, pull the thread horizontally from the top of the spool. Obviously, the spool rolls toward you.
- Now, pull the thread horizontally from the BOTTOM of the spool. While it may seem at first that this would cause the spool to roll away, it actually rolls toward you! (see description for an explanation of this).
- OPTIONAL: Show that the behavior changes at a certain angle, where you pull so as to create no torque about the pivot (see description).