The amazing spool
If you pull something, it always moves towards you. Right? Well, not always happens what you would expect on first glance. In this experiment we show you how you can determine in which direction a spool of thread rolls despite the fact that you always pull the thread from the same side of the spool.
Materials you need for this experiment:
spool of thread | For this experiment you need a spool with a border at the outside. If you don't have such a spool, you can easily build one by using the cardboard role of toilet paper and glue two circles of cardboard (like wheels) to both ends of the role. Make sure that the diameter of the wheels is at least 1 cm bigger than the role. The bigger the difference in diameter, the easier the experiment can be done. |
How to do the experiment:
 | Pull the thread at a steep angle. In which direction does the spool roll? |
 | Pull the thread at a shallow angle. Does the spool role in the same direction? |
 | If you pull the thread at a steep angle, you create a pull force that goes in the same direction as the pulled thread (see image on the left). This causes a positive (=counterclockwise) rotation of the spool which means that the spool moves away from you. Put a bit less scientifically: you simply unwind the thread from the spool. |
 | If you pull the thread at a shallow angle, you create a pull force that goes in the other direction (see image on the left). This causes a rotation in the opposite direction (clockwise) and the result is that the spool also moves in the opposite direction. You pull the spool towards you! |
Can you determine what is the critical angle for pulling the thread so that the spool rolls either toward you or away from you? What is the influence of the difference between inner radius (of the role where the thread is rolled on) and outer radius (of the "wheels") on the critical angle?
This experiment you can also use as a magic trick: If you pull the thread very near the critical angle, a minimal change in the height from which you pull decides whether the role moves towards you or away from you.
Have fun experimenting!