Frequency and Period:
The Basics of Wave Behavior
Name: ______Block: ______
Introduction
Have you ever watched a looping video on the Internet, where the same short film piece plays over and over? You might notice that the video lasts so long, then resets and plays again.
Watch the following short movie (gif), and answer the questions:
https://vine.co/v/iI7PdilVK2D
a) How long does the video short last? (use units)______
b) How many times does the video short play in one minute? ______
Frequency is a concept that deals with how often something happens. Period looks at how long the event takes. We are going to do some activities in class today that will help you understand the difference between the two and how we use them in science.
Class Demonstrations and Observations
Bicycle Tire with bright index card
1) How long does it take (period) for the card to do one revolution? ______
2) How many times does the card go around in one minute (frequency)?______
Flying Pig
1) How long does it take (period) for the pig to do one revolution? ______
2) How many times does the pig go around in one minute (frequency)? ______
Giant Pendulum
1) How long does it take (period) for the pendulum to make one complete trip back
and forth? ______
2) How many times does the pendulum go back and forth in one minute (frequency)? ______
3) What would happen to the period and frequency if we shortened the pendulum?
What if we made it longer? ______
a. Explain what happened:______
______
Snakey
1) How long does it take (period) for the pulse to make one complete trip back
and forth? ______
2) How many times does the pulse go back and forth in 5 seconds (frequency)?______
Complete each of the following sections below using your computer
Masses and Springs (PHeT) - https://phet.colorado.edu/en/simulation/mass-spring-lab
Slide the friction slider to 0. Select the middle spring, and put a 250g mass on it. Drop the top of the mass from the dashed line. Click the ¼ speed button, then use the stopwatch to time one period.
1) How long does it take (period) for the 250 g mass on the spring to make one
complete trip away from and back to the dashed line?______
2) How many times does the 250 g mass on the spring go up and down in 1 minute (frequency)? ______
Motion that repeats itself is called harmonic motion. What we have been seeing are oscillations. Objects that make repeated motions are called oscillators.
Observe this wave machine built by the Citadel School. After watching the entire clip, write down three detailed observations of the patterns of movement that you saw. Draw a picture in your explanation. - http://www.youtube.com/watch?v=1M8ciWSgc_k
Measuring Frequency
A cycle is a unit of motion that repeats itself over and over. A period is the time (measured in seconds) for one cycle. The unit Hertz (Hz) is used to measure 1 cycle/second, and is used to measure frequency.
Here are some examples of frequencies you may know: ______
______
Other Examples:
Heartbeat = ½ - 2 cycles/sec = ½ - 2 Hz
Musical note “A” = 440 cycles/sec = 440 Hz
Human voice = 100-2000 cycles/sec = 100-2000 Hz
Cell phones = 100 million cycles/sec = 100 MHz (somewhere between radio and microwaves, closer to radio waves)
101 FM radio station = 101,000,000 cycles/sec = 101 MHz
Microwave oven = 2.4 billion cycles/sec = 2.4 GHz
Frequency, Pitch, and Hertz - http://www.youtube.com/watch?v=fGAz7nfTpe4 – stop at 1:55
1) How is the frequency of a wave measured? ______
2) How do they measure the amount of Hertz? ______
3) The shorter the wavelength, the higher the ______, which in turn means the higher the ______.
a. Name three examples of this from our exploring waves lab: ______
______
4) When a sound wave has a high pitch, it also has a high ______. If a sound wave has a low pitch, it also has a low ______.
Oscillating Movements and Particles
Let’s look at a video of Japanese wave pool. - https://www.youtube.com/watch?v=PAZGkbJHzSY
1) What is the period of oscillation of a single particle in the wave pool? ______
2) How many waves come through in 30 seconds? ______
3) Describe the motion of the swimmers when the wave passes through. Do they move up and down, side-to-side, a combination of both? Explain. ______
______
Now, let’s now look at water waves more specifically. http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
4) Browse down to the third animation labeled Water Waves. What are the two types of waves in a water wave? ______
5) Describe the motion of the yellow particles close to the surface. ______
______
Wave on a String
Let’s look at the Wave on a String simulation of the PHeT site.
https://phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.html
Turn “damping” down to zero. Click on “oscillate”. Turn Frequency down to 22. Turn the “rulers” and “timers” on. Click “No End”. Click ‘Pause’.
1) What is the wavelength of the wave? ______
Change the frequency to 50. Click ‘Play’. Then ‘Pause’ to measure.
2) What is the wavelength now? ______
Change the frequency to 100. Click ‘Play’. Click ‘Pause’.
3) What is the wavelength now? ______
4) What happened to the wavelength as the frequency increased? ______
5) If I wanted the wavelength to be half of its current size, what would I need to do to the frequency? ______
The more peaks in the same amount of time; means it has more cycles per second, which means it has a higher frequency.
Speed (s) = Frequency (f) x Wavelength (λ)
6) Say the speed was constant through a medium. What would happen to the frequency of the wave if the wavelength increased? ______
Calculating Speed, Frequency and Period
Speed (s) = Frequency (f) x Wavelength (λ) Speed (s) = Distance (D) / Time (t)
1. The wavelength for a wave on the string of a wave generator machine is 0.5 meter, and its frequency is 40 Hz. What is the speed of the wave? FWAU
2. The frequency of a wave is 300 Hz, and has a speed of 20 m/s. What is the wavelength of the wave? FWAU
3. The speed of a wave on a slinky is 5 m/s, and has a wavelength of 1.5 meters. What is the frequency of the wave? FWAU
4. A wave travels down a slinky that is stretched to 12 meters. It takes a pulse 2.5 seconds to travel down to the other end of the slinky, bounce, and come completely back. What is the speed of the wave? FWAU
4. What is the frequency of a pendulum that is oscillating back and forth with a period of 1.6 seconds per oscillation? FWAU
5. What is the period of a cowboy with a lasso around his head that is whirling the lasso with a frequency of 0.5 Hz? FWAU
6. What is the frequency of an ocean wave that has 5 waves pass a buoy every 8 seconds? FWAU
7. The period of a wave is 10 seconds and the wavelength is 2 meters. What is the speed of the wave? FWAU
8. If it takes a wave on a string on the wave generator machine 0.25 seconds to make one complete oscillation, what is the frequency reading on the timer box? FWAU
Frequency in Your Home - Homework ______/12 Name ______
Many electronic devices use frequencies that are measured in Hz. Others are: 1) Kilohertz (KHz), where frequencies involve thousands of cycles/sec; 2) MegaHertz (MHz), where the frequencies involve millions of cycles/sec); 3) GigaHertz (GHz), where frequencies involve billions of cycles/sec.
Look at some electronic devices in your home. You will find the number of Hz on a tag on the device, usually on a silver tag under or behind the device.
Device Hz Number
1.______
2.______
3.______
4.______
5.______
Other items I found:
Period (T) and frequency (f) are the reciprocal of each other. This means that they relate to each other in an indirect way. For example, if the frequency increases, then the period has to decrease.
Example:
A drummer hits a drum 2 times in a second (frequency). The length of time between each hit is .5 second (period). 2 hits = Each hit is .5 seconds long (adds up to 1 second).
The same drummer hits a drum 4 times in a second (frequency). The length of time between each hit is .25 seconds (period).
4 hits = .25 seconds + .25 seconds + .25 seconds + .25 seconds (adds up to 1 second).
So the following equations can be used:
Period = 1_____ OR T = _time_(sec) frequency =___1_____ OR f = _# of cycles_
frequency # of cycles period time (sec)
Frequency of a wave is how many cycles (vibrations or oscillations) a particle goes through in one second. Measured in Hertz (Hz).
Period of a wave is how much time it takes a particle in a substance to make one complete oscillation (vibration) or cycle. Measured in seconds (sec).
Practice Problems:
1) A drummer hits a drum 4 times in one second. What is the period of each drum beat? FWAU
2) A dragonfly flaps its wings back and forth 121 times each second. What is the period of the wing flapping? FWAU
3) As a basketball coach, Mr. Gabel paces back and forth along the sideline 10 times in 2 minutes. What is the frequency of his pacing in Hz? FWAU
4) Non-digital clocks, which are becoming very rare, have a second hand that rotates around in a regular and repeating pattern. What is the frequency of the rotation of a second hand on the clock in Hz? FWAU
5) A common Physics lab involves the study of oscillations of a pendulum. If a pendulum makes 33 complete back-and-forth cycles of vibrations in 11 seconds, what is its period? FWAU
6) What is the frequency of a wave that makes 3 complete oscillations in 10 seconds? FWAU
Speed (s) = Frequency (f) x Wavelength (λ)
7) If a wave has a speed of 5 nanometers/sec and has a frequency of 10 Hz (cycles/sec), what is the wavelength of the wave (in nanometers)? FWAU