Physics / Unit 7: Waves and Sound
7.2 Wave Properties
Notes
Every wave can be described by a few measurements.
- Amplitude: maximum displacement from rest. Larger amplitude means more energy.
- Wavelength $\lambda$: distance between two neighboring crests (or compressions), in meters.
- Frequency $f$: waves passing per second, in hertz (Hz). Period $\displaystyle T = \frac{1}{f}$.
- Wave speed: $v = f\lambda$. The speed depends on the medium, not on the frequency.
- Example: a wave with $f = 4 \text{ Hz}$ and $\lambda = 2.5 \text{ m}$ travels at $v = 4 \times 2.5 = 10 \text{ m/s}$.
Practice
A wave has a frequency of 50 Hz. What is its period?
$\displaystyle T = \frac{1}{50} = 0.02 \text{ s}$.
Waves on a lake travel at $3 \text{ m/s}$ with a wavelength of 6 m. What is their frequency?
$\displaystyle f = \frac{v}{\lambda} = \frac{3}{6} = 0.5 \text{ Hz}$.
A student shakes a rope faster, doubling the frequency. The wave speed in the rope stays the same. What happens to the wavelength?
$\displaystyle \lambda = \frac{v}{f}$; with $v$ fixed, doubling $f$ halves $\lambda$.
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