College-Prep Physics: First, we brainstormed possible factors that might affect the maximum strength of static friction between two surfaces. Then students designed their own experiments to determine which of those factors actually mattered. Finally, we tried to use our “interlocking bristles” model to explain our results.
— Weight/mass: Definitely affected friction. Why? The bristles interlocked more, making it tougher for them to slide past each other. This is easily demonstrated and felt using toothbrushes.
— Surface Area: Surprisingly, this did NOT matter (with the exception of groups that used highly irregular surfaces like carpet, felt, and cork). Why? Well, a larger surface area means more bristles in contact, which should mean more friction. But a larger surface area also means the surfaces are less compressed, which would reduce the the friction. This is easily demonstrated with weights and foam.
So the net effect is no change in friction.
— Surface material: Changing the material of either surface also affected the maximum amount of static friction between the surfaces. This is similar to changing the material and arrangement of the toothbrush bristles.
NGSS Science and Engineering Practices
#2. Developing and using models
#3. Planning and carrying out investigations