Day 61: You learn something new every day

Today we were investigating how friction works as a class. We had talked about static and kinetic friction way back when we introduced forces in the first place, now it was time to find the similarities and differences between them. We started with magnitude comparison in my first class. I have cardboard ‘sled’ that I add mass to, then pull with a rope attached to a force detector. The force detector really reads the tension in the rope, but we talked about  (complete with FBD’s) that when the sled is at rest or moving at a constant velocity, the tension must be equal to friction. Thus the detector gives us the value of friction, for those cases.

I started the demo, pulled gently until the sled moved, then pulled it at a constant velocity. The graph looked very much like the top one on the picture above. I asked when the sled started moving, someone said ‘before the peak…the peak is the accelerating part.’ Challenge accepted. I wanted to show, as textbooks do, that the peak of the graph is when the object starts moving. So we grabbed a motion detector. We hooked it up, redid the trial, found the time when the object started moving based on the velocity graph, and found that….the object started moving significantly before the peak, as shown above. Hmm. Let’s try that again. Same thing. Wow. We broke physics. Although, in most cases, at least the force at the time of movement was still greater than the force while it moved at a constant velocity…but not convincingly so.

As often happens in class, I didn’t have a good explanation at the time (I am a processor…I come up with great comebacks to what people say to me, 5 minutes after the leave).  But it turns out that I continue to increase the pull even when it starts moving. Seems obvious now, as the time between moving and the max force is 0.15 seconds, similar to human reaction time.

I tried another trial where I increased the pull quickly, that certainly didn’t help. But then I tried increasing very slowly (this was after school was out when I had a couple minutes to play).

It was important to be able to feel the breaking point happen, as I was increasing my pull so little that the sled moving allowed the pull to decrease at that instant. Nice to know that physics still works. It’ll be fun to discuss this with the class tomorrow!


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