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Ch. 11, Vibrations and Waves

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# Ch. 11, Vibrations and Waves - PowerPoint PPT Presentation

Ch. 11, Vibrations and Waves. A repeated motion, like the pendulum on a clock, is called periodic motion . Another exp is a mass on a spring. At the equilibrium position, the speed is at its maximum. At the equilibrium position, the acceleration is zero.

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## Ch. 11, Vibrations and Waves

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### Ch. 11, Vibrations and Waves

A repeated motion, like the pendulum on a clock, is called periodic motion.

• Another exp is a mass on a spring.

At the equilibrium position, the speed is at its maximum.

• At the equilibrium position, the acceleration is zero.
• Friction causes a damping force that causes an oscillator to stop.
Hooke’s Law
• Felastic = -kx
• Negative sign indicates that the direction of the spring force is always opposite the direction of the displacement.
• This can be released as kinetic energy.
Pendulums
• Contain a mass called a bob, attached to a fixed string.
• The net force acting on a bob is Fg,x = Fg*sin(theta)

As long as a pendulum stays within an angle of about 15 degrees, it is a very good example of SHM.

Page 374, Figure 5, Gravitational Potential increases as a pendulum’s displacement increases.

• Table 1, page 375.

f = 1/T or T = 1/f

• T = 2*pi*(L/9.81)^.5
WAVES
• What are Waves? Discussion

Most waves are called Mechanical Waves. They need a medium to travel through.

• Some waves do not require a medium, like electromagnetic radiation waves.

Pulse Wave: One wave

• Periodic Wave: Continuous waves

Trough A low point

• Crest A high point
• Wavelength (lambda)  Distance between a whole cycle.

Frequency = number of complete waves that pass a certain point per second.

• Period = How long it takes ONE wave to pass a certain point.
Speed of a wave
• v = f*lambda
• Hence, two or more waves can occupy the same space at the same time.

Constructive interference: Two or more waves on the same side of the equilibrium position combine and form a bigger wave.

Destructive interference is the superposition of two or more waves that are on opposite sides of equilibrium. A smaller wave results.

Reflection: a wave is reflected at a free end; a wave is reflected AND inverted at a fixed end.

Standing Wave: occurs when two equivalent waves interact in opposite directions.

• Best example is with a rope attached to a wall.

Areas of largest amplitude: Antinode.

• Areas of complete destructive interference: Node.
Computer Assignment
• Find an example of SHM (best bets are springs or pendulums). Provide:
• Picture
• A problem, with walkthrough on how to solve it.