Physics Lab: Waves on a String
Waves are a familiar part of everyday life. You have probably seen water waves and waves on a string. You hear sound waves and see light waves.
Waves have a wavelength, λ, a frequency, f, and a speed, v. These three quantities are related by:
v = fλ
In this lab you will look at a simulated wave on a string – a transverse wave. You will set the frequency of the wave and then measure the wavelength and speed. We will use these to try to answer the question: “Does wave speed really equal frequency times wavelength?”.
We will also look at reflection of waves.
Required Materials
Waves on a String
Procedure
Go to the simulation by clicking on the link: Waves on a String.
If the link does not work, copy and paste the following address into a browser:
https://phet.colorado.edu/sims/html/wave-on-a-string/latest/wave-on-a-string_en.html
You should see this: At the bottom of the screen, set Damping to None. Tension should be set on High.
Check the boxes for Rulers and Timer.
At the upper right click on No End.
Click Slow Motion. It will be easier to measure the speed of a wave when it is moving slowly. The timer will also be slowed down so the time measurements will be accurate.
Note the controls on the bottom:
You can pause (and start) the motion.
You can go forward in discrete steps.
You can reset to the initial conditions.
You can drag the rulers and timer around (we will only use the horizontal ruler).
The timer is a stopwatch. You can start, stop and resetIn the upper left click Oscillate.
You should see something like this:
At the bottom set the frequency to 3.00 Hz. Once you have a stable wave going, pause it and measure the wavelength with the ruler. You need to measure the length from one peak to the next. Enter your value in the table below in the row headed by 3.00 Hz. Be sure to include units with every entry in the table.
Note that as you go through this experiment the wavelength will be getting longer. At some point you will not be able to see an entire wavelength. When that happens, you need to measure the horizontal distance from a peak to a trough (low point) and double the result to get the wavelength.
Now measure the speed of the wave. Start with a peak at the beginning. You can get to this position by using the step button. See the following image:
Use the timer to find how many seconds (to two decimal places) it takes for that starting peak to move 5 cm. Measure carefully! Enter your time in the table.
Calculate the speed (distance divided by time) and enter in the table (include units).
Calculate frequency times wavelength and enter in the table. Note that Hz is really inverse seconds so the result of your calculation will have length units divided by seconds.
Are speed and f the same? Calculate a percent difference by taking the absolute value of the difference between the two and dividing by fλ:
Now repeat this process for f = 2.50 Hz, 2.00 Hz, 1.50 Hz, 1.00 Hz and 0.50 Hz.
f λ distance time v fλ %difference
3.00 Hz 5 cm
2.50 Hz 5 cm
2.00 Hz 5 cm
1.50 Hz 5 cm
1.00 Hz 5 cm
0.50 Hz 5 cm
Do your results confirm that v = fλ? Explain your answer by referring to your data Reflection of a Wave
Now go to the simulation and uncheck Rulers and Timer.
Click Fixed End and Pulse.
Set Damping to None.
Push the pulse button once and observe its motion as it reflects back and forth along the string.
Now click Loose End.
Observe the motion of a pulse in this setting.
Describe how the fixed end and loose end waves are similar and how they differ.
