Module 7—Oscillatory Motion

Explore

 

When you walk or run, your arms swing. You don’t mean for this to happen, but it does. Without you thinking about it, your arms swing at a given frequency that feels natural to you and is based on your walking movement. This “natural” frequency is known as a resonant frequency. Any object or system that oscillates or vibrates does so at a unique frequency that depends on the physical characteristics of that object. For example, your arms probably don’t swing at the same frequency as your friend’s arms when you are walking side by side. (You could make them do this, but it will not feel natural.) And your arms will not just oscillate on their own; they have to be disturbed or caused to swing by your walking motion. A pendulum is an excellent example of this. (Your arm is, in fact, a sort of pendulum.) As you learned in the previous lesson, if a pendulum is disturbed, it swings back and forth with a constant period that is defined by the physical length of the pendulum. If you change the length, you change the period and the frequency at which the pendulum swings. Therefore, the physical size of the pendulum is related to its resonant frequency.

 

However, without a force, your arms will not swing and a pendulum would hang motionless. A force is required to create the oscillations, and it has to be present to maintain the resonant frequency once the motion starts. The pendulum in a clock, for example, has to be “wound” occasionally. When this is done, a spring mechanism or a hanging weight in the clock is used to keep “pushing” the pendulum. This keeps it oscillating as friction acts to slow it down. This is similar to the “pumping” action you would exert on a swing set—as soon as you stop pumping your legs, the swing slows down and eventually stops. And you have to pump at precise moments on a swing—legs completely extended on the way up and feet tucked under on the way down. Any deviation from this pattern causes the swing to slow down and eventually stop. The pumping has to match the resonant frequency of the swing.

 

The pumping in this example is called a forced frequency. If this frequency is close to or matches the resonant frequency of the swing, the swing will oscillate with a large amplitude. This is known as mechanical resonance.

 

 

Watch and Listen

 

See how a child on a swing illustrates mechanical resonance by watching the streaming video resonance on swing. Look for the answers in the video to the following two questions.

Self-Check

 

SC 1. A child on a swing is a kind of _____________.

 

 

Module 7: Lesson 3 Assignment

 

Remember to submit the answer to TR 1 to your teacher as part of your Module 7: Lesson 3 Assignment.

 

Try This

 

TR 1. The periodic push that a child exerts on the swing must match the ____________ frequency of the swing.

 

Watch and Listen

 

See how mechanical resonance is used in musical instruments by viewing the streaming video resonance in music. Look in the video for the answers to the following four questions.

Module 7: Lesson 3 Assignment

 

Remember to submit the answers to TR 2 and TR 3 to your teacher as part of your Module 7: Lesson 3 Assignment.

 

TR 2. Resonance stems from the Latin noun meaning _________________.

 

TR 3. Every oscillating system has a ___________ frequency, which is determined by the ______________ properties of the object.

 

Self-Check

 

SC 2. In the piano, the vibrating strings make the soundboard ____________.

 

SC 3. Small vibrations at the resonant frequency create a __________ amplitude vibration.

 

 

Read

 

Read “Applications of Simple Harmonic Motion” on pages 381 to 383 of your textbook.

 

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