4.7 Friction  (Page 4/12)

Take-home experiment

An object will slide down an inclined plane at a constant velocity if the net force on the object is zero. We can use this fact to measure the coefficient of kinetic friction between two objects. As shown in [link] , the kinetic friction on a slope $f_{\text{k}}={\mu }_{\text{k}}\text{mg}\text{cos}\theta$ . The component of the weight down the slope is equal to $\text{mg}\text{sin}\theta$ (see the free-body diagram in [link] ). These forces act in opposite directions, so when they have equal magnitude, the acceleration is zero. Writing these out:

Solving for ${\mu }_{\text{k}}$ , we find that

Put a coin on a book and tilt it until the coin slides at a constant velocity down the book. You might need to tap the book lightly to get the coin to move. Measure the angle of tilt relative to the horizontal and find ${\mu }_{\text{k}}$ . Note that the coin will not start to slide at all until an angle greater than $\theta$ is attained, since the coefficient of static friction is larger than the coefficient of kinetic friction. Discuss how this may affect the value for ${\mu }_{\text{k}}$ and its uncertainty.

Making connections: submicroscopic explanations of friction

The simpler aspects of friction dealt with so far are its macroscopic (large-scale) characteristics. Great strides have been made in the atomic-scale explanation of friction during the past several decades. Researchers are finding that the atomic nature of friction seems to have several fundamental characteristics. These characteristics not only explain some of the simpler aspects of friction—they also hold the potential for the development of nearly friction-free environments that could save hundreds of billions of dollars in energy which is currently being converted (unnecessarily) to heat.