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Q & A: weight and variable friction

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Most recent answer: 09/26/2013
I have always been taught that the mass value will not affect the acceleration of a block sliding down an incline. But in the lab using vernier motion detectors and sliding cylindrical weights down a track covered with aluminum foil, we repeatedly measured a larger acceleration with the heavier masses. Why does this happen? Is it possibly because the masses are moving at slow enough speeds that it is in the transition regime between static and kinetic? I am quite frankly puzzled by the results. We did not get good clean data. There was not a good straight line for the velocity vs time, but even to the naked eye the heavier mass moved much more quickly. In order to have a larger velocity at the bottom it had to have a greater acceleration.
- Kimball Clark (age 57)
Gallilpolis, OH USA

We love questions like this about physics and the tangible world.

Your thoughts on the possible transition toward the static friction case are very interesting, but I suspect that the sliding may be too fast for that to be significant. There are a couple of other ways that what you were taught could be incorrect. There's a little air friction, and that should be more significant for the acceleration of the lighter weight. This effect also seems likely to be too small to be so noticeable. Most importantly, the rule about how the sliding  friction is a fixed coefficient times the normal force is just a rule of thumb. It may be inaccurate here. 

That raises the question of how to test the possibilities. Here's some thoughts.

If the static/sliding transition is the main effect then the accelerations should start off different but then become quite close, once the speed gets out of the crossover range. You might be able to tell that already from your data, or could find out by using longer tracks.

If air friction were the main effect, since it grows rapidly with speed you'd find that the accelerations were mainly different toward the end of the runs, not the beginning.

If it's that the rule about f=μN is wrong here, then the accelerations will probably differ pretty consistently from start to finish.

You might try something similar with a teflon surface. That would leave the air friction effect unchanged, but change the other two effects. You could also reduce the slope of the surface. That should make an especially big difference for the sliding/static transition effects. There are bound to be still other things you could change to sort this out.

Mike W.


(published on 09/26/2013)

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