A Friction Experiment with Inclined Planes Michael C. Faleski Delta College Oct. 11, 2008

Common Activity Determine coefficient of static friction between an object and a surface. Experiment: Place object on surface Raise angle of surface, Θ, until object slides. Result from FBD: μ = tan Θ

Question: So…… What is the coefficient of static friction between an object and a surface if the object is….

Standard Results Rolling without Slipping: Acceleration: with Slipping: Acceleration:

Neat result… Plot Rolling without slipping: –Slope: 5/7 Intercept: 0 Rolling with slipping: –Slope: 1 Intercept:

Experiment Set angle of track to desired value Release object from the top of track. Places photogates a known distance apart to time object when released from rest. Analysis:  Compute average time for object to move between photogates  Use constant acceleration kinematics to compute acceleration of object.

Preliminary Results QuantityTheoryExpt (2 sets) Slope (rolling)5/7 (0.714)0.816(11) Slope (slip)11.13(2) Intercept (slip)-.157(21)

Problems Ball not triggering photogate at instant of release (need to account for this!) –This could be controlled experimentally… and there is a simple way to compute the acceleration knowing the distance. Rolling friction not included in analysis. Thickness of track for object can play a roll –very small here, but see Adam Niculescu, “A Rolling Sphere Experiment,” Phys. Teach. 44, 157 (March 2006).

Assume that the ball started 3.0 mm from photogate QuantityTheoryExpt (2 sets) Slope (rolling)5/7 (0.714)0.732(10) Slope (slip)11.003(26) Intercept (slip)-.140(22)

Conclusions Simple Experiment for finding friction coefficients Derivations can be adjusted for level of student –AP/IB students derive relations for object rolling without slipping Rolling friction… not considered… does it matter? Improvements – lots of places!! –Challenge: Improve the design to obtain better results! –New releasing mechanism and/or fix release distance