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Lesson: Chapter 5 — Friction

5.3. What limiting friction depends on 1 of 4

Activity 02 — roughness

Friction acts between the surfaces that are in contact, so it is reasonable to suspect that three things about them might matter: how rough they are, how much of them touches, and how hard they are pressed together. Three activities, one for each — starting with roughness.

Apparatus

A wooden block of weight 60 N, a Newton balance, and several sheets of sandpaper of different grades of roughness.

Figure to be added

Figure 5.3 — the block with sandpaper stuck to its lower surface, the rough face against the table, and the Newton balance held horizontally.

Method

  1. Take the wooden block and stick the least rough sheet of sandpaper to it so that its lower surface is completely covered.
  2. Stand the block on the table with the rough surface of the sandpaper against the table, as in figure 5.3, and pull the balance horizontally — first with a small force, then increasing it gradually.
  3. Take the reading on the Newton balance at the instant the block just begins to move. This is the limiting friction.
  4. Now take a rougher sheet of sandpaper, stick it to the lower surface as before, and find the limiting friction again.
  5. Repeat with several sheets of different roughness and compare the results.

Observation

As you move from the least rough sheet to progressively rougher ones, the limiting friction increases in the same order.

Conclusion

The roughness of the contact surfaces does affect friction. Note that the block, its weight and the area touching the table were all kept the same — only the roughness was changed, which is what lets us blame the roughness for the difference.