Shake one end of a cord. How fast does the wave run, and how long is each wave?

What you'll discover

• A wave carries energy along the cord, but the cord itself only moves up and down.
• Shaking faster makes shorter waves; the speed along the cord stays the same.
• Wave speed equals frequency times wavelength.

The science in plain words

A wave is a disturbance that travels, carrying energy from one place to another without carrying the material along with it. When you flick a cord, each bit of cord pulls the next bit up, then down, so the shape moves along while each piece stays in place, moving only up and down. This sideways jiggling makes it a transverse wave.

Waves have a few key measurements. The wavelength is the distance from one crest to the next. The frequency is how many waves pass a point each second, measured in hertz. The amplitude is how far the cord moves from its resting position, and bigger amplitude means more energy.

These are linked by a simple rule: wave speed equals frequency multiplied by wavelength. The speed depends on the cord itself: how tight and how heavy it is. So if you shake faster, the frequency goes up and the waves become shorter, but they run along at the same speed.

The same rule works for sound, water waves, light and radio. Learning it on a cord you can see makes all those invisible waves easier to understand.

Where it fits

• India, NCERT: Class 9 science, Chapter 11: Sound
• United States: Grades 6 to 8
• Next Generation Science Standards: MS-PS4-1

Common questions

What is a wave in science?

A wave is a moving disturbance that carries energy from one place to another without carrying the material with it. Ripples on a pond and vibrations on a guitar string are both waves.

What is the formula for wave speed?

Wave speed equals frequency times wavelength. A wave with a frequency of 5 hertz and a wavelength of 2 metres travels at 10 metres per second.

What is the difference between transverse and longitudinal waves?

In a transverse wave, such as on a shaken rope, the material moves at right angles to the wave's direction. In a longitudinal wave, such as sound, it moves back and forth along that direction.

Related labs

• Sound Waves: A speaker squeezes the air. Watch the squeezes march across the stage.
• Standing Waves: Buzz a cord at just the right rate and it splits into still points and big loops.
• Wave Reflection: Ripples hit a tilted wall. Which way do they bounce?

See all physics labs or every Taz science lab.

Try it yourself

The Wave Motion lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.

Try this lab on Taz