It’s amazing what you can come across at a car boot sale. I am always on the lookout for a bargain that can be used in my classroom, and at a recent sale, I came across a Slinky. For those of you who were not blessed with the ownership of one of these toys as a child, it is a giant spring that has the fascinating ability to ‘walk’ down stairs or a steep slope.
For me, it is the perfect visual aid to help explain how energy from an earthquake travels as seismic waves in all directions through the earth. There are basically 2 types of wave – body waves (that pass through the earth itself) and surface waves (that travel at the surface of the earth).
Body waves can be classified as either Primary waves or Secondary waves. Primary waves – also known as ‘P’ waves, or ‘push’ waves, or ‘compressional’ waves – are the first shock waves to arrive. When a ‘P’ wave passes through rock, the rock particles shake backwards and forwards in the direction in which the wave travels. Like sound waves, ‘P’ waves can pass through solids and liquids – so if any part of the earth’s interior is liquid, then they are able to pass through it.
To use the Slinky to demonstrate ‘P’ waves, simply extend the slinky on the floor or on a desktop, get a student to hold one end firmly, and then at the other end, pull the Slinky towards you and rapidly push it away. This gives a straight push along the line of the Slinky. You will see parts of the spring compress, and parts dilate as the wave moves. In other words, you have recreated a longitudinal wave where the particle oscillation and wave propagation are in the same direction.
Secondary waves – or ‘S’ waves, or ‘shear’ waves, or ‘shake’ waves – are the second waves to arrive, and these work in a different way. When they travel through rock, the particles vibrate (shake) at right angles to the direction in which the waves move. Unlike ‘P’ waves, ‘S’ waves cannot pass through liquids, meaning that if any part of the earth’s interior is liquid, ‘S’ waves will stop at the liquid boundary. ‘S’ waves travel about half as quickly as ‘P’ waves, but are more violent and cause most of the damage in an earthquake.
To demonstrate ‘S’ waves, extend the slinky as before, and move your hand from side to side to make the wave travel down the spring in a sinuous way like a snake slithering along. In a ‘shake’ wave, the direction of displacement is perpendicular to the line of wave propagation.
Surface waves (or ‘long’ waves) are restricted to the surface of the earth in much the same way that waves on the sea are restricted to water near the surface. In this respect, ‘long’ waves differ from ‘P’ and ‘S’ waves which can travel through the interior of the earth, and are therefore known as body waves.
To demonstrate the surface long waves of an earthquake, the slinky needs to be moved to display a shifting from left to right (so called ‘Love’ waves) or a shifting from up to down (so called ‘Rayleigh’ waves). To combine the two, the hand holding the slinky can be moved in a circular motion, imparting a rolling wave down the Slinky.
Another way of showing the effect of seismic waves is to use the students themselves in a physical demonstration. To recreate a ‘P’ wave, form a line of students at the front of the class, or outside. Get them to stand one behind each other and face forward. They should then extend their arms and put their hands on the person in front, locking their elbows. Their solid arms represent atoms, the chemical bonds that hold solids rigidly together. As a ‘P’ wave is a motion in the same direction as the wave is travelling, give a firm shove to the last person in the line (warn him or her first!), representing the release of energy at a rupture in a fault plane. This energy will then be transferred through the atoms in the line of students – representing a ‘push’ wave. When ‘P’ waves move through a liquid, the atoms can move independently. So, in the same line-up arrangement, get the students to leave a gap between their hands and the shoulders in front. After applying your ‘push’, the same energy flow results.
To demonstrate how ‘S’ waves operate, get the line of students to turn and face the front. Ask them all to put their hands on their hips and link arms. They are still representing a solid with chemical bonds. In an ‘S’ wave, the vibrational motion of atoms is perpendicular to the direction of the wave, so give the end person a good sideways shake to send the energy flowing down the line. If you repeat the experiment with unlocked arms (representing the atoms not being chemically bound), you can show how shear waves cannot pass through a liquid.
You can also show how ‘P’ and ‘S’ waves travel at different speeds. If you ask all the students to put their ear to their desk, you can get them to hear a sound travelling up through the desk when you drop a large object on the floor. Next, get a student to drop a large object on the floor in the corridor outside. They should hear it first through the desk, and then a split second later through the air. This is of course, due to the fact that sound travels faster through solids than it does through the air. This is why seismic waves reach a point at different times, ‘P’ waves travelling faster than ‘S’ waves.
My bargain hunting also takes me (much to Mrs B’s displeasure) into charity shops and pound shops – a great source of cheap Play-Do, post-its, and of course the geographers favourite – colouring pencils! So, get hunting in your local shops and car boot sales for resources of your own. There are often loads of books, maps, globes, compasses and so on to be found, as well as old board games that can be adapted (by students) to a particular geographical theme. Let me know what treasures you discover!