Question

Describe an experiment to show friction between two solid surfaces.

Answer 1

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.

Drag the block along with the forecmeter

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.