Activity 03 — area of contact

Now the factor almost everybody gets wrong. A bigger face on the table surely means more friction — more of the block is touching, after all. The experiment says otherwise.

Apparatus

A wooden block of weight 60 N whose length, breadth and height are all different, a Newton balance, and several sheets of sandpaper of the same roughness.

Method

Take the block and stick sandpaper on the faces of different area. The block is not a cube, so its faces A, B and C are genuinely different sizes.
Stand the block with its largest face touching the table and find the force needed to start it moving — the limiting friction.
Then, as in figure 5.4, stand it on each of the other faces in turn and find the limiting friction for those too.

Figure to be added

Figure 5.4 — the same block resting on face A, on face B and on face C in turn, each pulled horizontally by the Newton balance.

Observation

Whichever face is against the table, the limiting friction comes out the same.

Conclusion

The area of the contact surfaces does not affect friction. It is the one factor in this section that makes no difference, and it is worth remembering precisely because it feels as though it should.

Why the sandpaper had to be the same

Activity 02 has just shown that roughness matters. If the faces carried different grades of sandpaper, a change in the reading could be blamed on either factor and the experiment would prove nothing.