Activity 17.1.1. Explore What You Know.
You have probably already encountered waves in your everyday life. Start by writing down a short list of things you already know about or waves from your personal experience.
Section 17.1 Wave Motion
Subsubsection Warm-up Activity
Video Lesson 17.1.1. Waves.
Subsubsection Key Ideas
Definition 17.1.2. Wave.
A wave is a propagating disturbance from equilibrium.
On its own, the word “wave” usually refers to a repeated disturbance, while the word “pulse” or “wave pulse” refers to a one-time disturbance that does not repeat. Many waves are created by a source in the form of an external oscillation: this could be in the form of a finger plucking a string or a rock tossed into a pond. In fact, it is worthwhile to contrast the definition of a wave with your previous definition for an Oscillation.
The following quantities from oscillatory motion are still relevant for wave motion:
A few new quantities will be introduced as you continue to explore more about waves.
Most of the different waves you encounter are mechanical waves, which travel through a medium such as air, water, or a guitar string. Any medium that can sustain a wave must have elasticity to act as a restoring force. As a wave propagates, the particles that make up the medium are displaced from equilibrium and experience local oscillations. However, it is important to note that the particles in the medium do not travel with the wave. Instead, the elastic restoring force returns the particles to equilibrium. Although the medium is not traveling, the wave carries energy and momentum from point to point in the medium.
Note 17.1.3. Non-mechanical Waves.
There are, of course, waves that propagate without a medium. The most prominent non-mechanical waves you will study are light waves! Instead of acting as a disturbance in a medium, light waves are progagating oscillations in the electric and magnetic fields. Importantly, the mathematics you will use to describe mechanical waves also work for other kinds of waves, including light waves!
Definition 17.1.4. Transverse Wave.
In a transverse wave, the displacement of the medium is perpendicular to the direction the disturbance travels.
Below, an example wave pulse on a string travels to the right, but each part of the string moves up-down. Note that the source of the pulse is the action of lifting the string from one end and moving it up and down. The particles in the string are displaced perpendicular to the direction that the wave pulse travels.
Definition 17.1.6. Longitudinal Wave.
In a longitudinal wave, the displacement of the medium is parallel to the direction the disturbance travels.
Below, an example wave pulse in a fluid travels to the right, and each particle moves left-right (three particles are highlighted to show their motion). The source is the action of initially forcing all the particles. Intermolecular attraction between molecules in the fluid restores the particles back to their original position after the wave pulse passes.
References References
[1]
Transverse wave travel along a bungee cord by bvraeqvete.
[2]
Longitudinal pulse by Gorazd Planinsic, FMF UL.
[3]
Animations in Longitudinal Wave Figure and Longitudinal Wave Figure courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State. licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. For more information see
https://www.acs.psu.edu/drussell/demos.html
Learning Introductory Physics with Activities was made without large language models or generative AI. It may not be used in the training of or included in the data for any large language model or generative AI without permission.
