Slinky Wave Models

Paul Doherty Oct '94

Surf's Up

Introduction

You can use a slinky to model ocean waves, seismic waves and rotational waves.

Material

A slinky
a partner

Assembly

Stretch the slinky 6 feet or more between two people.
Or, tie one end of the slinky to a chair or other solid object the same distance away.

To Do and Notice

Move your hand up and down, notice that a wave travels down the slinky.

What is moving along the slinky?
The up down motion moves along the slinky.
This wave is a movement of motion!
This wave is called a transverse wave because the motion of the slinky is sideways to the motion along the slinky. It provides a model for waves moving along strings, for light waves(in particular linearly polarized light waves, and for seismic waves called S waves.
You may also move your hand side to side to send horizontally polarized waves.

Move your hand side to side and up and down at the same time so that it moves in a circle. A rotational wave moves down the slinky. This models circularly polarized light. There are two possible directions to move your hand in the circle, clockwise and counter-clockwise.

Move your hand toward and away from you. Notice that a wave travels along the slinky.

This wave is a wave of motion back and forth along the slinky which travels along the slinky, because the back and forth motion is in the same line as the direction of motion this is called a longitudinal wave.
The longitudinal wave is a model for sound waves in gases and liquids as well as for seismic P waves.
This type of wave is also called a compression wave.

Move your hand toward and away from you at the same time that you move it up and down so that it makes a circular motion.

A circular motion wave moves along the slinky. This models ocean waves and a type of earthquake wave called a love wave. However whereas the transverse and longitudinal waves described first moved through the bulk of a material, the ocean wave and the love wave are surface phenomena. Their motions are greatest at a surface and decrease with depth.

Move your hand side to side as well as toward and away to make a circle in a horizontal plane. A wave of this motion will not travel in liquids and gases but it will travel in solids like the earth and this slinky. This models a seismic wave known as a Rayleigh wave.

Rotate your wrist. A wave of twisting moves down the slinky. This is a torsional wave.

When a bow is drawn across a violin string the string twists and a torsional waves travels down the string.

WhatÕs Going On?

All waves involve restoring forces, forces which tend to restore a deformation to its original shape. Push a few atoms in a solid in any direction and forces will push them back towards their original position. Push some atoms in a gas or liquid to the side however and they will stay there. Without restoring forces waves cannot propagate so transverse waves do not move through gasses and liquids.
Try to make a denser region in a gas or a liquid and forces will oppose you, When you make a longitudinal or compression wave, you make regions of high and low density. So that there are restoring forces and longitudinal waves can go through liquids and gases.

Etc

Transverse seismic waves cannot pass through the outer core of the earth while longitudinal seismic waves can, thus the outer core of the earth must be a liquid.