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A rigid cubical block A of small mass M and side L is fixed on top of an identical cube B of low modulus of rigidity eta, with A's lower face fully covering B's upper face; the lower face of B is clamped to a horizontal surface. A small force F applied horizontally to the top side face of A makes the side of B shear through an angle phi. The whole assembly is now inverted so that A's lower face is clamped to the surface and the same force F is applied horizontally to the top side face of B. Through what angle does the side of B now turn?

1. phi
2. phi/2
3. phi/3
4. zero

Correct answer: zero

Solution

In the inverted case the deformable block B is on top and the force acts on it directly; with A rigid and clamped, no shear deformation is produced in B in the manner that tilts its side, so the angle is zero.

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