Passage I

A science class studied the frictional forces involving moving surfaces.

In each of the following experiments, the students used identical bricks that were either unwrapped or that were securely wrapped in paper. The students tied a string around a brick, fastened a spring scale to the string, and pulled the brick across a table, as shown in Figure 1.

The amount of the force, in newtons (N), required to move the brick across the table at a constant speed was measured on the spring scale. This value equaled the amount of the frictional force between the brick and the tabletop.

(Note: A brick's facets consist of 2 faces, 2 sides, and 2 ends, as shown in Figure 2. Unless otherwise stated, each brick lay on one of its faces. Also, unless otherwise stated, all bricks were pulled across the table at the same low speed.)

Experiment 1

Four bricks were each wrapped in a different type of paper. Each wrapped brick was pulled across the table. The measured forces are given in Table 1.

Experiment 2

The brick that was wrapped in brown paper was pulled across the table while the brick lay on a face, a side, and an end (see Table 2).

Experiment 3

An unwrapped brick was pulled across the table. The procedure was repeated with 1, 2, 3, or 4 identical unwrapped bricks stacked facedown on top of the first unwrapped brick (see Table 3).

Experiment 4

The brick that was wrapped in brown paper was pulled across the table at a constant medium speed and then at a constant high speed (see Table 4).