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Waves

Waves. Physics H. Pendulum. A pendulum is simply a mass (bob) suspended from a string that can swing back and forth The time it takes for the pendulum to swing back and forth is called the period. Period depends on length of the pendulum, not on weight suspended.

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Waves

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  1. Waves Physics H

  2. Pendulum • A pendulum is simply a mass (bob) suspended from a string that can swing back and forth • The time it takes for the pendulum to swing back and forth is called the period. • Period depends on length of the pendulum, not on weight suspended. • The back and forth motion is called simple harmonic motion (SHM) • Produces a sine curve

  3. The restoring force of a pendulum is a component of the bob’s weight • x-component pushes the bob toward equilibrium • At small angles, a pendulum follows SHM • At large angles this breaks down • Potential energy increases as displacement increases • Cons of energy still applies PE + KE = const.

  4. The following holds true for a pendulum or a spring At the equilibrium point, v is a max At max displacement, force and “a” are max In SHM restoring force is proportional to displacement Hooke’s Law Felastic = -kx k = spring constant x = displacement - = force is always opposite direction from displacement See Figure 12-1 A stretched or compressed spring has elastic potential energy Hooke’s Law

  5. Sample Problem 12A • If a mass of .55kg attached to a vertical spring stretches the spring 2.0cm from it’s equilibrium position, what is the spring constant? • Not accelerating so Nnet = 0 • Fnet = 0 = Felastic - FW

  6. -kx – mg = 0 • Rearange k = -(mg)/x • k = -(.55kg)(9.8m/s2)/(-.02m) • k = 270 N/m

  7. The period of a simple pendulum can be calculated with period = 2Π x square root of length / gravity Period of a mass spring system in SHM period = 2Π x square root of mass / spring constant Pendulum Mass-Spring

  8. Ex • When a mass of 25g is attached to a certain spring, it makes 20 complete vibrations in 4.0s. What is the spring constant? • Calculate period first • 4.0s/20vibrations = .2s / vibration • Now use the formula

  9. .2s = 2Π√(.025kg/k) • .1s/ Π = √(.025kg/k) square both sides • .00101s2 = .025kg / k • k = 25 N/m

  10. Sample Problem 12B • You need to know the height of a tower, but darkness obscures the ceiling. You note that a pendulum extending from the ceiling almost touches the floor and that its period is 12 s. How tall is the tower?

  11. Sample Problem 12C • The body of a 1275 kg car is supported on a frame by four springs. Two people riding in the car have a combined mass of 153 kg. When driven over a pothole in the road, the frame vibrates with a period of 0.840 s. For the first few seconds, the vibration approximates SHM. Find the spring constant of a single spring.

  12. Homework • 12A-C odd

  13. Wave Motion • Mechanical waves travel by molecules vibrating back and forth or up and down. • Sound, ocean wave • They have to travel through a medium • Water, air • Pulse – single traveling wave

  14. Transverse Wave - Motion of the wave is at right angles to the direction the wave is moving. Longitudinal Wave - Motion of the wave is in the same direction as the direction the wave is moving. Wave Types

  15. Waves are made up of several parts Crests - high points Troughs - low points Amplitude - distance from the midpoint to the crest or trough Wavelength - distance form the top of one crest to the top of the next. Frequency - How often a vibration occurs. Period - amount of time for 1 cycle Wave Description

  16. Wave

  17. Frequency • Frequency measures the number of times a wave oscillates in a given time (second) • Frequency is measured in hertz • 1 cycle per second = 1 hertz • frequency = 1/period • period = 1/frequency

  18. Wave Speed • Wave speed depends on what type of medium it is traveling through • The speed of a wave is dependant upon 2 things, wavelength and frequency • Wave speed = wavelength x frequency • v = f

  19. The Sears tower swings back and forth at a frequency of .1Hz what is its period? What is the wavelength of a 170Hz sound wave when the speed of sound in air is 340m/s? Frequency = 1/period .1Hz = 1/period Period = 10s v = f 340m/s =(170Hz)  = 2.0m Examples

  20. Sample Problem 12D • The piano string tuned to middle C vibrates with a frequency of 264 Hz. Assuming the speed of sound in air is 343 m/s, find the wavelength of the sound waves produced by the string.

  21. Interference • Occurs every time two waves overlap • Constructive- When crests of two waves overlap, it results in increased amplitude. The waves are said to be in phase • Destructive- When the crest of one wave overlaps with the trough of another wave, they cancel each other out. The waves are said to be out of phase with each other.

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  23. Reflection • When ever a wave transfers from one medium to another, part of the wave reflects and part of the waves energy continues on. • At a free boundary, waves are reflected. • At a fixed boundary, waves are reflected and inverted.

  24. Standing Waves • If a string attached to a wall vibrates at exactly the right frequency, it can produce a standing wave. • Standing Wave- A wave pattern that does not move along the string. • Node – There is no motion on the string • Antinode – midway between the nodes, vibrations have the largest amplitude

  25. What are the two types of interference? How fast is a wave with a wavelength of 3m and a frequency of 212Hz moving? Describe the two types of waves. If a wave has a period of .2s, what is its frequency? A radio station has a frequency of 101 MHz, what is the period of its wave? What are the two points of importance on a standing wave? Review

  26. Chapter 12 Review • p. 469 #8-9 • p. 470 #19-22 and 35 • p. 471 #36

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