Some months ago, I saw a You Tube video where a rock guy (not music) had big concrete blocks and bungy cords and I figured I could replicate this as a practical experiment in the classroom.
The pic taken with my phone shows the set-up …………
1. a wooden block with medium grade sandpaper glued to its base.
2. a sheet of similar sandpaper taped to the bench
3. a spring balance and an elastic band attached to a hook on the block
4. a ruler to measure displacement
The students were told to pull on the spring balance as slowly as they could, to get the highest possible reading on the scale, before the block jerked forwards. They were to record the force on the scale when the block moved, and then measure the displacement. They did 10 trials using the same equipment, and then graphed the results.
Here is one of the graphs ………
Displacement is shown as x and force as o
The red line represents a reasonably consistent force for each of the 10 trials, but the displacement was very erratic.
The force (provided by the elastic band) represents the pressure on the tectonic plates caused by the convection currents operating in the asthenosphere. I was hoping this would prove to be fairly constant.
However, the displacement was erratic, sometimes the force (approximately 4 newtons) caused a displacement of over 25 cm and other times, less than 10.
Does this mean that earthquakes will sometimes be over 6 on the Richter Scale and sometimes less than 4?
Results seem to support the hypothesis that slippage, caused when a tensional force overcomes friction, is somewhat unpredictable, despite the fact that the force remains relatively constant.
This can be applied to the real situation of earthquakes. Although earthquakes are bound to occur in some parts of the world, the intensity (as governed by the degree of slippage) is unpredictable.
Did this give the students an understanding of why we can’t predict when the next big earthquake will happen? I think so.
There are lots of ways we could vary the set-up from here……… bigger blocks (more surface area), heavier blocks, finer/rougher sandpaper etc,
If anybody wants to try this and some of the variations ………please share your results.