# Friction between solid surfaces

##### Demonstration

This is an exploration of which factors affect the force of friction when one surface slides across another and which do not.

#### Apparatus and materials

Forcemeter, 30 N, large size for easy class viewing

Plank wit screw eye 91 m x 15 cm x 1 cm), smooth

Block with screw eye (25 cm x 12 cm x 1 cm), smooth

Blocks without screw eye (25 cm x 12 cm x 1 cm), 3

Rollers, 1 cm diameter and 20 cm long, 10

Crank assembly

G-clamp to fix crank assembly to bench

#### Health & Safety and Technical notes

Dimensions are for guidance and are approximate.

The faces of the block must be of equal smoothness.

#### Procedure

a Place the plank on the bench and drag the block along it with the forcemeter. It is difficult to obtain a fixed forcemeter reading. Repeat the process a number of times. Estimate the average force that is needed to pull the block. This is equal to the force of friction acting between the surfaces of the block and the plank.

b Use a crank to pull the plank so that it slides beneath the block. This will let you make a more reliable measurement of the force of friction. Place the plank on rollers. Pull it along at a constant speed using the string and crank.

c Ask students to predict the effect of dragging the surfaces across each other at different speeds. Show them what happens.

d Press down against the plank with a finger and again drag the plank along. This increases the frictional force between the surfaces of the plank and block. The forcemeter shows an increased force.

e Add an equal block on top of the first so that the force pushing the block on to the plank is doubled. This force acts at 90° to the surfaces. Call it the 'normal' force. Measure the frictional force. Increase the load with two, three and four blocks and see how the frictional force increases.

f Ask students to predict the effect of change in contact area of the surfaces. Turn the block on its side and drag it as before to demonstrate what happens.

#### Teaching notes

1 In b, the block is held at rest by the forcemeter while the plank moves under it. The frictional force is balanced by the tension in the forcemeter.

2 Students should be able to discuss conclusions about the force of friction and three other variables: speed, area of contact, and the normal force. The force of friction is independent of area of contact (provided that the surfaces are equally smooth) and of speed (at low speeds). It is proportional to the normal force.

3 Normal force: Since you have varied this by using the weights of 1, 2, 3 and 4 blocks, a simple quantitative conclusion is possible.

Ask students if doubling the normal force has the effect of doubling the frictional force, or whether the behaviour is more complex than that. You could go further, asking whether they believe that the frictional force always changes by the same proportion as the normal force, thus bringing in the idea of 'proportionality' of variables.

A graph of frictional force (y axis) against normal force (x axis) provides a visual presentation that quickly reveals the nature of the relationship. However, measurements in this activity are subject to a fairly high degree of uncertainty, or 'error'. Students' conclusions should acknowledge this uncertainty.

4 You can show that force of friction is greater between static surfaces than between moving surfaces. Leave the plank at rest, not on the rollers, and just pull the block. The forcemeter reading increases from zero up to a maximum value before the block slips and the reading falls.

5 Try placing sheets of different materials (wood, plastic, paper) between the block and the plank. (With a bit of ingenuity you can also try ball bearings, plastic beads and/or oil, using suitable tracks.)

6 How Science Works Extension: This experiment provides a good opportunity to discuss the nature of variables. As described above, the experiment looks at a categoric variable (the nature of the surfaces) and a discrete variable (the normal force, determined by the number of equal blocks). Contact area is also a discrete variable, since it is altered by turning the block on its side.
Discuss whether these variables could be changed to become continuous variables. Students should suggest that, by adding weights to a single block, any value of normal force can be obtained. This then opens the way to a discussion of how more values of contact area can be obtained. Blocks of different areas could be made and the contact force adjusted to a standard value by adding suitable weights.

You can then discuss the independent, dependent and control variables in each part of the experiment. See our general guidance on Variables

This experiment was safety-checked in October 2004