Factors Affecting Friction
Friction is vital to the safe landing of airplanes. In this photograph, a test pilot lands on an unplowed runway to see if the plane can come to a stop without skidding off the runway.Copyright NASA
Based on what students have already learned about friction, they formulate hypotheses concerning the effects of weight and contact area on the amount of friction between two surfaces. In the associated activities (Does Weight Matter? and Does Area Matter?), students design and conduct simple experiments to test their hypotheses, using procedures similar to those used in the previous lesson (Discovering Friction). Their data analyses reveal the importance of weight to normal friction (the friction that occurs as a result of surface roughness) and the importance of surface area to the friction that occurs between smooth surfaces due to molecular attraction. Students also use their data to calculate coefficients of friction for the tested materials and compare these to published values for various materials.
Engineers must understand how friction affects a number of situations, from the bottom of skis in which friction is a disadvantage to hiking boots where friction provides traction. Scientists must think like engineers when designing experiments as the they do in both of the associated activities.
After this lesson, students should be able to:
- Describe the effects of weight on normal friction, that is, the friction due to surface roughness.
- Describe the effects of contact area on the friction that occurs as a result of molecular attraction.