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Traveling Waves: Nuts & Bolts

Traveling Waves: Nuts & Bolts

3. Nuts & Bolts

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When solving a traveling wave problem you will most likely be using the wave equation $D(x,t) = Asin(kx - \omega t)$. Remember that $k = \frac{2 \pi}{\lambda}$ and that $\omega = 2 \pi f$ where $\lambda$ is the wavelegnth and $f$ is the frequency of the wave.

Graphical problems:

Draw a physical representation of your system and label any quantities given in the problem statement. It is a good idea to either draw a sketch of $x$ versus $t$ or an overhead snapshot of the situation, depending which is appropriate - for example, for the motion of a particle in a vibrating string, $x$ versus $t$ is more appropriate; for analyzing ripples on a pond radiating from a point, the overhead view will be more appropriate. Label wavelengths and/or periods.

Graphical problems will typically ask you about how many cycles will pass a point in a given amount of time, or how many wavelengths will fit in some area. Remember that a wavelength is a distance in meters that tells you how far a wave must travel to be at the same point in its cycle; the period is a time that tells you how long a wave must propogate for before it returns to the same point in their cycle. 

 

Other problems:

Identify the type of wave - is it mechanical or electromagnetic? If it is an electromagnetic wave then you immediately know that its traveling with speed $ c = 3 * 10 ^ 8 $ m/s, and you can quickly find the wavelength if given the frequency, and vice-versa. Recall that $v_{wave} = f \lambda$. A common question when considering traveling waves is how long it takes a wave to travel from point A to point B. Understanding the wavespeed makes this problem easy - the distance is just $d = v t$.

 All of the mathematical operations inside of the cosine/sine function can make the wave equation look intimidating, but are simple to handle case-by-case. For example, say you have the following equation - $D(x,t) = A sin( 30x - 50t) $ - and you are asked what are the wavelength and period of the wave. A good way to remember which terms are associated with wavelength and frequency is to inspect the equation and notice that there is an $x$, which we typically measure in meters, and $t$, which we measure for time. Since we want the argument inside of a trig function to be dimensionless, we know that $x$ is associated with wavelength and $t$ with the frequency or period.

Since the wave equation is $D(x,t) = Asin(kx - \omega t)$, we know that in the previous example

$$k = \frac{2 \pi}{\lambda}$$

$$⇒ \lambda = \frac{2 \pi}{30} = 0.21 m $$

and 

$$ \omega = 2 \pi f $$

$$⇒ f = \frac{\omega}{2 \pi} = 8 s^{-1} $$

$$ T = \frac{1}{f} = 0.125 s $$

 

Multiple Representations

Multiple Representations is the idea that a physical phenomena can be explored in many different ways. For example, there is the physical representation which models the system with figures and diagrams, such as a free body diagram. There is also the mathematical representation which uses the equation(s) governing the physics of the system. All of the representations can be used together to help us understand and quantify the physical phenomena.

Observe the different types of representations for this section below:

Physical Representation

Physical Representations explain the features of the situation in a visual way, often with vector representations of physical quantities overlaid a simple diagram or picture of the situation.

 

There are two basic traveling waves: Logitudinal, Transverse. Traveling waves may be a combination of both types. Longitudinal waves travel in a line, whereas, transverse waves propogatae up and down.

 

Mathematical Representation

Mathematical Representation uses equation(s) to describe the situation.

$\nu = f \lambda$

$\kappa = \frac{2 \pi}{\lambda},\; \omega = \frac{2 \pi}{T}$

$\nu_{string} = \sqrt{\frac{F^{T}}{\mu}}$ 

$\nu_{sound} \propto \sqrt{Temperature}$

$x(t) = \pm x_{max} \frac{\sin \text{or}}{\cos} (\kappa x \pm \omega t)$

Velocity ($\nu$), Frequency ($f$), Wavelength ($\lambda$), Wave number ($\kappa$), Angular frequency ($\omega$),

Linear mass density ($\mu$), Force tenstion ($F^{T}$), Period ($T$)

Graphical Representation

Graphical Representation describes the situation through use of plots and graphs.

This is a snapshot graph that depicts a a wave at different points in time.

A history graph describes the history of a specific position with respect to time

Descriptive Representation

Descriptive Representation is made up entirely of words or annotations. Think about how you might explain the situation to someone else.

 

Traveling waves are comprised of longitudinal and transverse waves. Longitudinal waves propogate in a line, whereas, transverse waves propogate up and down. Traveling waves may also be comprised of both types of of basic waves. For example, electromagnetic waves are a combination of both transverse and longitudinal traveling waves.

Experimental Representation

Experimental Representation could be thought of as doing the experiment.

Longitudinal and transverse waves experiments are easily performed using a slinkly for longitudinal waves or a rope for trasnverse waves. Check out the videos below to see the experiments. 

Longitudinal 

Transverse

 

Example Problems

  • The Physics Classroom, good set of problems Website Link
  • Openstax's section on Traveling Waves has great practice problems, Website Link

For additional practice problems and worked examples, visit the link below.