Predict/Explain You push two identical bricks across a tabletop with constant speed, v, as...

Predict/Explain You push two identical bricks across a tabletop with constant speed, v, as shown in Figure. In case 1, you place the bricks end to end; in case 2, you stack the bricks one on top of the other. (a) Is the force of kinetic friction in case 1 greater than, less than, or equal to the force of kinetic friction in case 2? (b) Choose the best explanation from among the following:

FIGURE

I. The normal force in case 2 is larger, and hence the bricks press down more firmly against the tabletop.

II. The normal force is the same in the two cases, and friction is independent of surface area.

III. Case 1 has more surface area in contact with the tabletop, and this leads to more friction.