First, I, the moment of inertia, of the main stick was found as is explained on the Data Spreadsheets page. The goal was to determine if friction is a significant force in the use of Devil Sticks. In order to determine this, a predicted acceleration was needed to compare to the collected data. In a perfect hit to the stick, the force applied is perpendicular to the main stick, thus using no friction. This is very difficult to obtain on every hit, so friction appears to be necessary.

The force of gravity is acting on the stick at the point of the yellow arrow, labeled F.g, downwards. This is the center of mass. The applied force is acting at the point indicated by the red arrow, marked F.a, upwards. This point will be called the point of contact.

The second challenge of this experiment was to answer the question of where to hit the stick for best performance. This plays a role in the friction challenge as well. As is evident in the graphs with foil of y-direction acceleration, the distance of contact from the center of mass makes a significant difference as noted below the graph. The closer to the center, the more the applied force needed to accelerate the stick. This is because when hit so close to the center, most of the force converts into upward force unless the stick is rotated fast enough. If hit directly at the center, the stick will not rotate at all, and will just be pushed upward. On the opposite end of the spectrum, hitting it closer to the end has the reverse effect. When hit here, the force mostly rotates the stick. In order to keep the stick in the air, and counteract gravity, a longer acting force must be applied to keep the center of mass from falling. If hit quickly at this location, the stick would spin away and applying the upward force on it would be very difficult.

As the instructions mention, ¾ seems to be the best place to hit the stick to easily maintain perpendicular hitting and keep the stick going. The above picture shows this location. The video clips demonstrate this.

To first address the friction question, intuition and playing with the sticks was used. If not hit perpendicularly, the stick slides if there is no friction. There are a few hits in the foil video of this sliding. However, in the trials without foil, the rubber held the stick from slipping down the control stick. This makes it much easier to play with the Devil Sticks.

Our data supports our intuition about the large amount of help friction provides. In the trials without foil, as can be seen from the graph of acceleration in the x-direction, there appears to be a large source of error. This is due to assuming that friction is not present. Acceleration was derived from torque in this case. The torque assumed the force of the control stick was perpendicular. If the hit is not perpendicular, there is a force exerted on the stick due to friction by holding the stick up. On the other hand, if friction is negligible, the stick slides down the control stick.

In conclusion, friction is a very important aspect of Devil Sticks. Less friction makes them more challenging and more difficult to perform tricks. For a beginner, friction is very useful to assist the person when they are learning to be precise with their hits. The ¾ mark is also a useful fact to know when learning.
 
 

Last Updated 1030h.9.6.2002