In these three tasks, you are going to use a brick being pulled along a surface covered in sandpaper to model the behaviour of an earthquake:
Turning the pulley to build up tension in the string is like the build up of stresses at a fault, and the brick movement over the sandpaper is like the slippage that happens in an earthquake.
You will compare your results with real earthquake data and evaluate the brick and sandpaper as an earthquake model.
Task A: Looking at the build-up of forces
Make sure the pulley is clamped onto the plank and attach the string from the pulley onto the force meter.
Use another string attached to the force meter to tie around the brick.
Stick or tape a ruler with a millimetre scale onto the plank and a pointer onto the brick: you should be able to measure the position of the brick against the scale to the nearest mm.
Start the brick at the end of the sandpaper furthest away from the pulley: make sure it is completely on the sandpaper.
Wear eye protection. Turn the pulley so it gradually increases the tension in the string (and the force on the brick) until the brick starts to move. Increase the tension slowly so that it takes several seconds before the brick slips.
Try this a few times. Watch what happens to the force meter reading: does the brick always begin to slip when the force reaches the same value?
Task B: Looking at the amount of slip
Pointer position (mm)
0 (Start position)
Place the brick back at the start position. This time, measure how far the brick moves each time it slips. Record your results in a table like the one shown here.
Continue until you have at least thirty readings.
Plot a histogram to show the frequency for each size of slippage.
What do you notice about the relative frequency of large slippages?
Compare your histogram with a histogram showing the frequency of different magnitudes of earthquake.
Task C: Patterns, predictions and models
If you have access to a data logging kit with a force sensor, you could investigate some or all of the following questions:
Does the force return to a particular value after every slip?
Do bigger forces lead to bigger slippage?
Is there a critical level of force which triggers slippage?
Is there a relationship between the force drop and the size of slippage?
What patterns does your earthquake model show? Can we use it to predict when an 'earthquake' will happen or how big it will be?
Compare your results to standard earthquake models.