Friction: Horizontal Plane

Equipment:

  • Clamp pulley
  • Variety of wooden blocks with different surfaces
  • Variety of different weights and weight holders
  • Variety of boards with different surfaces
  • String
  • Weighing device

Objective: To explore the concept of limiting friction

Setting up the experiment

  • Secure the pulley to a desk
  • Weigh your block of wood
  • Place the block of wood on the desk attached to a length of string.
  • Place a weight on the other end of the string such that the block does not move.

Your task

  • Increase the weight on the string to the point where the block is just about to move.
  • Repeat the experiment with different weights on the block of wood.
  • Record your results. Think about the information you need to record in order that that others can test your results
  • From your results estimate the coefficient of friction.

Explore how the coefficient of friction changes as different surfaces are used. In each case predict what you think the coefficient of friction is likely to be before you begin to experiment.

Teacher Notes

The aim is for students to be able to model the following situation and be able to use F =µR to find the coefficient of friction.

Students should be encouraged to repeat the experiment several times using progressively heavier weights on the block. A value for µ should be calculated for each trial. From observing the variations in the values of µ students should be able to estimate a value for the coefficient of friction for the particular surfaces being used.

Students should be encouraged to predict what weighs produce limiting friction using their value for µ and then check experimentally and comment upon their findings.

Students can repeat the activity using different surfaces. It is good practice to estimate what value for µ they expect to find with different surfaces, even if this is just a higher or lower value than for the last surfaces used.

Different groups can compare their results and comment upon the similarities and differences of their results.

Probing questions

  • What would happen if µ=0?
  • Will µ always be between 0 and 1 or can µ be greater than 1?
  • Can µ ever be negative?
  • If you conducted this experiment, using the same equipment, with surfaces with µ=100 what would you expect to happen?

Follow up questions