FRICTION

Jim Szeszol                    Mendel Catholic  High School
                               250 East 111 Street
                               Chicago, IL 60628
                               1-312-995-3700

OBJECTIVES: 
     To study the effects of friction  in a simple mechanical system. 

APPARATUS: 
     1) Self propelled dynamics cart (coil spring driven)
     2) Meter stick
     3) stopwatch
 
Procedure: 
     1) Perform "Hookes Law" experiment for coil spring constant
     2) Determine distance and time for PE transfer
     3) Calculate KE transfer for
          a) Translational KE
          b) Rotational KE  - wheels, pulleys, axles
     4) Determine the efficiency of the system
     Calculations : Determine the difference in energy levels using KE formulas 
for energy of translation and rotation. The  ratio of kinetic energy of the cart 
to the potential energy of the spring will yield an efficiency value for the 
system. 
     Conclusion : In most first year physics courses energy lost to friction is 
traditionally ignored. Through this activity the observation can be made that a 
significant (80% - 90%) of the system energy is lost to frictional forces while 
doing useful work. 

Extensions / Modifications : 
     For translational KE only keep the ratio of rotational mass to translational 
mass small. 
example - small wheels - light wheels - single pulley - added mass to cart.
     For non-uniform motion - the cart will accelerate at rates dependent on the 
coil spring constant and the selected step pulley. To study these rates a timer 
tape can be attached to the cart to determine varying rates of motion.