What it shows
A long coiled spring or a stretched coil of telephone cord can carry two kinds of waves: sideways shakes make transverse waves and push-pull shakes make longitudinal waves. In both, the wave travels along but the coils only move back and forth.
The science
A wave carries energy from one place to another without carrying matter along. In a transverse wave the particles move at right angles to the direction of travel, as on a rope; in a longitudinal wave they move back and forth along the direction of travel, forming compressions and rarefactions, as sound does in air. The speed of a wave on a spring depends on how tightly it is stretched, and a pulse that reaches a fixed end reflects back.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A long soft coil spring toy, or a telephone handset cord, or a few metres of thick rope
- A smooth floor or long table
- A piece of ribbon to tie to one coil as a marker
- A stopwatch
- A measuring tape
- A helper
Safety
For this project: Do not overstretch the spring or let go of it suddenly, because it can snap back and hurt. Work on the floor with enough space and keep faces away from the ends.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Stretch the spring about 3 m along a smooth floor with your helper holding the other end, and keep it fairly tight.
Tie the ribbon to one coil in the middle as a marker.
Give one quick sideways flick at your end and watch the pulse run along to your helper. Notice that the ribbon only moves sideways and does not travel along with the pulse.
Now push your end quickly forward and back along the spring, and watch the compressed coils travel along. The ribbon moves forward and back along the spring.
Time the pulse for the 3 m stretch with a stopwatch, repeat five times and calculate the speed as distance divided by time.
Stretch the spring tighter, repeat the timing and compare speeds.
Watch the pulse reflect from your helper’s fixed end and come back. Notice that it comes back upside down for a sideways pulse.
Draw both waves with labels for crest, trough, compression and rarefaction.
What you should see
You see a sideways pulse and a push-pull pulse travelling about 2 to 6 metres per second, faster when the spring is stretched more, while the ribbon moves only locally.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the difference between transverse and longitudinal waves?
In transverse waves the particles move at right angles to the wave direction; in longitudinal waves they move along it.
2Does the wave carry the ribbon along?
No, a wave carries energy and not matter.
3Which type is sound in air?
Longitudinal.
4What happens to the speed if the spring is stretched tighter?
The speed increases.
Take it further
Shake the spring at a steady rate to make a standing wave with nodes and measure its wavelength.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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