Resonance is easy to detect and explain. Resonance occurs when an object vibrates with its natural frequency due to input of energy.

Clear demonstrations are possible. Take two identical tuning forks and strike one with the other close by. On a count of three, hold the prongs of the tuning fork which was struck. What do you feel? What do you hear? How could you make this into a whole class puzzle activity? Observers or participants are certainly intrigued by this activity.

The two tuning forks must be of identical frequency for this to work and ideally should be mounted on wooden sounding boxes.

An analagous phenomenon is Bartons pendulums in which pendulums of identical frequency connected through a flexible string exchange energy while others cannot move effectively due to mismatched frequencies: like pushing a swing with the wrong tempo, some pushes slow down the motion and others miss the swing altogether.

Only those of identical frequency can exchange energy.

This can be set up by tying a string between two stands. Tie four or five lengths of string to the string so that they hang down. Tie weights onto each one and ensure that two only are the same length. Set one of the two identical pendula moving and watch what happens.

The simulation below allows the input of energy and shows how this can give rise to resonance but only for the correct frequency pendulum.

Resonance also plays a part in the tuning of radio receivers as the electromagnetic radiation induces ac currents in the receiver.

A related phenomenon occurs when air is blown into hollow tubes or when taut strings are plucked. In both cases, waves of energy move along the air in the tube or along the string. These then reflect from open or closed ends and the reflected waves then interfere with the original waves. Since the frequencies are identical, standing waves are produced.

The simulation of the superposition of waves also demonstrates consequent standing wave formation. In this case the waves appear to arise independently from opposite directions: just as is the case for reflected waves in instruments.

View this simulation carefully to see points appear at which there is no net movement at all: points of no displacement we call nodes. Other points between the nodes show maximal displacement and are called anti nodes.

These result simply from the superposition of the waveforms as the waves meet.

Find out what frequency multiples are produced by different strings and hollow pipes. Summarise the evidence from the class and suggest an explanation.

This simple animation helps visualisation. It shows how the particles move in longitudinal waves in a pipe with open or closed ends: just like musical instruments. It helps understand the echoes seen in the speed of sound measurements.