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[Phys-L] Free body diagram misconception



Hello all, I have a strange physics question that the 5 physics
teachers at our high school sat and pondered today. We have started
teaching AP-C physics and the teacher for this new course came to us
with a variation to a question from the book. Here it is.

Block 1 is at rest atop Block 2. Block 2 rests atop a frictionless
surface. A force is applied to Block 1 pulling it to the right. The
coefficient of friction between blocks 1&2 is some non-zero value less
than 1. What is the Net force equation for Block 2?

We drew the free body diagrams for both blocks and came up with the
following equation for Block 2.

(mu)m1g - F(pull) = m2a2

Now all of us agreed on this, but we also saw a problem. This means
that there is some value for the force of the pull that Block 2 will not
move. However it is on a frictionless surface and IF they were
connected the Frictional component would disappear - thus the object
would accelerate. What is wrong with our reasoning? Did we make a bad
assumption somewhere?

I love these problems but it is driving me bonkers since I know that I
am forgetting something fundamental --- Block 2 should accelerate for
ANY force pulling on it if it is on a frictionless surface.

Thanks in advance for any help.


Gene Gordon