How to play Slinky Lab
- Choose a test: ↕ is a sideways shake, ↔ is a push and pull. Read the numbers on the card.
- Think about which way the coils move, and use v = f·λ for spacings and speeds.
- Type your prediction and press Start. The slinky moves at half speed.
- Compare the orange line with the painted coil's white path on the right, or the orange marks and dot with the compressions.
Transverse and longitudinal waves
In every wave something travels while the material only moves about its place. What differs is the direction of that movement. In a transverse wave the material moves across the way the wave goes: a slinky or rope shaken side to side, ripples on water, and the S waves of an earthquake. In a longitudinal wave it moves back and forth along the way the wave goes: a slinky pushed and pulled, and sound in air, where molecules crowd into compressions and spread into rarefactions, and the P waves of an earthquake. Both kinds obey v = f·λ. In a longitudinal wave the wavelength is the distance from one compression to the next. A slinky is ideal for seeing both, because you can send either kind along the same spring and watch a single painted coil: it moves across in one, along in the other, and in neither case does it travel with the wave.
A focused learning model
The slinky is ideal: the wave keeps its shape and speed, and nothing reflects from the far end. Each coil follows u = A·sin(2π·f·(t − x ÷ v)) behind the front and stays still ahead of it. Sideways movement is 30 cm; movement along the slinky is 8% of a wavelength, which bunches the coils to about half their spacing. The painted coil sits a third of the way along. Runs play at half speed. Answers within 2% are right; along the slinky, both 0° and 180° count.