I haven't had a chance today to think analytically about this so please
don't flame me if I'm wrong, but watch the torques. It could be that when
you tilt the ramp, the weight on one axle is more than the weight on the
other, thereby increasing the friction of the former and leading to the
observed result. Did they also try the experiment with sliding blocks?
Introductory physics can be very subtle.
Vickie
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