Inclined Plane Test (Tilt Test)

The inclined plane test, illustrated in the figure, is often used to determine the static friciton coefficient for stable objects (objects that resist rolling in at least one orientation). The angle between the horizon and the inclined plane, ?, is increased until the object reaches a state of impending slip. The static friction coefficient is calculated using the tangent of ? and the known gravitational force acting on the object. However, a problem arises from this test: a sport-ball is not a stable object as we have defined one to be.

On the other hand, dynamic friction occurs between two solid surfaces whose relative velocity is not zero. As illustrated in Fig. 1, the resistive force remains seemingly constant as the applied force increases. It is important to note that relative velocity can affect the kinetic friction coefficient (Gordon, McGrew & Serway).

As a group, we conducted a tilt test in order to find preliminary values of what we expect the coefficient of static friction to be. This experiment was conducted in order to have experimental data to compare with the data that the Friction Tester yields.

As a group, we conducted a tilt test in order to find preliminary values of what we expect the coefficient of static friction to be. This experiment was conducted in order to have experimental data to compare with the data that the Friction Tester yields.

Note that there were ample sources of error. The tilting device was constructed in a matter of minutes, the surface was not ideal in that it was not perfectly uniform, the balls used were cheap and also not perfectly uniform, the crank which controlled the change of the angle ? turned imprecisely and vibrated (which would have caused the balls to begin slipping prematurely). Also, the slip angle was measured by eye and protractor so there was a degree of human error and imprecision in that as well.

The accuracy of this test is not ultimately important to the project. The test itself served chiefly as a lesson in the difficulty of running a valid tilt test.

Data

Photos

Tilt test in progress. John-Michael is turning the car jack to slowly change the angle, Dimitrios is prepared to catch the balls when they slip	Test from another angle. As John-Michael turns the crank on the jack, it slowly raises the angle of the surface.
Tilt test in progress	Dimitrios displaying the 3-ball configuration. With this configuration, the balls cannot roll - an important aspect of the tilt test!